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Psychological correlates of academic performance have always been of high relevance to psychological research. The relation between psychometric intelligence and academic performance is one of the most consistent and well-established findings in psychology. It is hypothesized that intelligence puts a limit on what an individual can learn or achieve.

Moreover, a growing body of literature indicates a relationship between personality traits and academic performance. This relationship helps us to better understand how an individual will learn or achieve their goals. The aim of this study is to further investigate the relationship between psychological correlates of academic performance by exploring the potentially moderating role of prior education. The participants in this study differed in the type of high school they attended. They went either to gymnasium, a general education type of high school that prepares students specifically for university studies, or to vocational school, which prepares students both for the labour market and for further studies.

In this study, we used archival data of psychological testing during career guidance in the final year of high school, and information about the university graduation of those who received guidance. The psychological measures included intelligence, personality and general knowledge. The results show that gymnasium students had greater chances of performing well at university, and that this relationship exceeds the contribution of intelligence and personality traits to university graduation. Moreover, psychological measures did not interact with type of high school, which indicates that students from different school types do not profit from certain individual characteristics. IntroductionSince the early days of psychology, prediction of academic performance (AP) has been of high relevance to psychologists ,.

This is due to the importance of AP in the life of every individual–it confines the range of possible job opportunities, as well as career choices. Higher educational level is often a prerequisite for more demanding jobs, which can also lead to greater financial outcomes. Moreover, a measure of academic success can also play an important role in the job application process, as a source of information about the candidate’s prior performance. Overall, it could be stated that the educational level achieved plays a role in the quality of life and well-being of an individual. Literature on psychological correlates of AP in post-secondary education indicates that there is a well-established relationship between intellectual abilities and AP , , that individual differences in personality can explain additional variance in AP , , and that general knowledge (GK) might be a valuable predictor of AP as well ,. Besides psychological individual differences, high-school success has traditionally been used in research to predict AP –, mostly operationalized as grade-point average (GPA) or as a result in standardized tests (such as SAT in the USA). High-school GPAs has also been widely used as university admission criteria.However, if a certain education system does not have standardized tests at the end of high school, GPA scores obtained by students from different high schools are hardly comparable.

In Croatia this type of school system was present prior to the year 2010, when the national state exam was introduced. Until 2010, all high-school students that were enrolled in 4-year programmes were eligible university candidates.

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We can make a broad distinction between two types of 4-year high-school programmes: gymnasium, which offers a general education aimed to prepare students for higher education; and vocational schools, aimed to prepare students for the labour market. However, successful leavers of both types of high school are allowed to apply for admission to universities.Therefore, in this paper, besides psychological predictors (intelligence, personality, GK), we explored the moderating role of high-school type in predicting AP. IntelligenceIntelligence is a general mental ability that reflects a broad capability for comprehending our surroundings, solving problems, planning, or learning from experience, while it does not reflect a set of narrow academic skills.AP has been the validity criterion for psychometric measures of intellectual ability , since their main objective in the early days was prediction of academic success or failure. Ever since, measures of intellectual abilities have become some of the most frequently-used psychological instruments, often used in employee selection , career guidance and clinical practice.Robust findings show that intellectual abilities are positive predictors of success in a variety of scholastic tasks, in level of education, and in work performance , –. Association between intelligence and AP differs slightly with respect to different measurement methods, and varies across the level of education in different studies. Correlation between intellectual abilities and AP declines with age, being highest at primary school (.60–.70), and lowest at graduate level (.30–.40) ,.

This decline is usually explained by the restriction of range in the university population, since fewer students continue education after high school. Moreover, intellectual abilities are associated with continuing to higher levels of education.Studies show that intelligence has high longitudinal stability in the period from childhood to early adulthood , , which makes it suitable for long-term predictions of AP at university.

For example, a longitudinal study showed that psychometric intelligence measured at age 11 makes a large contribution to scores in national examinations in 25 academic subjects at age 16. Verbal and numeric aptitudes measured in 7 th grade were moderate-to-high predictors of total grade and grades in four academic subjects in 10 th grade. Moreover, intellectual ability measured in the first year of university was in low correlation with academic success at the end of the first and third years of study. Personality traitsEven though some studies showed that intelligence explained more variance in academic achievement than did personality factors , a role for personality traits should not be excluded from the prediction of AP. It has been shown that both intelligence and personality can be related to successful learning.

In addition, different conceptualizations of intelligence and personality may also explain different aspects of academic performance. Whereas intelligence represents a set of specific abilities and puts a limit on what an individual can do, personality traits might indicate how an individual will do it.

Traditionally, academic performance has been considered to be more closely related to intellectual abilities than personality traits. However, abundant literature indicates that personality traits contribute to AP as well , –.

In this introduction, we further present relations between AP and Eysenck’s Gigantic Three personality factors: Psychoticism, Extraversion and Neuroticism (used in this study).Neuroticism has generally been shown to negatively predict AP and this effect decreases with academic level. It is argued that stress, impulsiveness and anxiety may influence AP (e.g. During the taking of an exam) in the same way they negatively relate to psychometric intelligence score , since both AP and IQ are measured with maximum-performance tests. It could also be possible that Neuroticism influences AP in other ways.

Neurotic students are more often ill during examinations, which might lower their AP. In addition, Neuroticism might direct a student’s attention towards anxious emotions, and away from academic homework.Correlations between Extraversion and AP are ambiguous across studies. It is expected that extraverts have higher levels of energy and generally more positive attitudes towards studying, which could be reflected in a desire on their part to acquire knowledge. However, extraverts benefit from this trait only at lower levels of education, where there is more interaction with teachers, and where visibility in class is appreciated. It is more likely that extraverts will favour socialization over studying, which might lower their AP at higher levels of education. In line with this, it has been shown that introverts outperform extraverts in secondary and tertiary education.

However, findings about the relationship between Extraversion and AP are inconsistent. For example, Petrides, Chamorro-Premuzic, Frederickson and Furnham reported negative correlation among a high-school sample, and Chamorro-Premuzic and Furnham reported positive correlation among a university sample, while Heaven, Mak, Barry and Ciarrochi reported no significant correlation among a high-school sample. Correlation size and direction vary relative to method of AP estimation , and relative to age and level of education.Psychoticism, the last of the Gigantic Three personality traits, has been systematically negatively related to AP in previous studies , –. Psychoticism is also negatively related to some other behaviours significant to academic excellence. For example, individuals high on Psychoticism found nothing wrong with school truancy , and had lower levels of responsibility and interest in studies , and lower involvement in coursework. General knowledgeGK represents one’s ability to acquire knowledge in general. It is not a clear measure of an individual’s cognitive abilities, and it is not explicitly related to a formal education.

The theoretical background of this construct may be inconclusive, since some researchers consider GK a first-order factor of crystallized intelligence , while others consider it a first-order factor of semantic memory. GK is positively correlated with general intelligence , and it could indicate how an individual uses mental abilities. Someone who is intellectually bright will never acquire broad knowledge if not devoted to learning, and if not in an environment that values education. On the other hand, an individual with moderate intellectual abilities, but with high motivation for learning, and in a supportive environment, will acquire more knowledge.Furnham and Monsen showed that a measure of general cognitive abilities based on general-knowledge questions was in low-to-moderate relation with academic grades. Furthermore, Furnham, Monsen and Ahmetoglu showed that a GK measure was in weak positive relation with English and Maths grades.

Since it has been shown that GK relates to both intellectual abilities and personality traits , it can be assumed that it may serve as a predictor of AP as well. Academic performanceAP has been measured in numerous studies, but researchers do not agree about its definition. Furnham and Chamorro-Premuzic proposed that this was due to familiarity with the concept. The simplest definition would be that AP is the success of individuals in formal education (elementary, secondary or tertiary education).

As with inconsistency in the definition of AP, researchers have conceptualized it differently in different studies: for example, as GPA , first-year examination scores , final-year examination scores , course performance or standardized PISA testing scores. However, researchers often investigate the relation of aptitudes or personality-trait scores to students’ scores in different tests, but rarely have information on students’ broader academic achievement, such as university graduation, which is a cumulative result of all the academic tasks that students have to fulfil prior to earning a degree. Type of high schoolIn most of the European Union countries, students may choose between general and vocational programs after finishing their primary education. In some countries, continuing to higher educational levels is limited after vocational-school graduation (e.g. In Germany vocational-school students have to take additional courses prior to higher-education admissions) , and in some countries they are allowed to enrol in tertiary educational programs.

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In Croatia, there are two types of high school–gymnasiums and vocational school–and students are selected for these schools based on their primary-school grades. The gymnasium programme offers a general education that qualifies students for university studies. Gymnasium graduates are not qualified for any profession, and it is assumed that they will continue their education at tertiary educational level. Vocational high schools offer programmes of 3 years (craftsmanship and industrial professions) and 4 years (medical, economic, agricultural professions etc.) that qualify graduates for specific professions. Four-year vocational programmes offer a mix of broad basic knowledge, as well as profession-specific knowledge, and graduates may apply for tertiary education studies (while 3-year programme graduates may not apply). About 60% of 4-year Croatian vocational school students continue to university education.

According to the Student Integration Model a key factor in successful university studies is the student’s integration within academic and social systems at the university. Individual characteristics that contribute to successful integration include the individual’s goal and commitment to achieving that goal, individual attributes regarding the importance of graduation, pre-college experience (usually GPA and academic and social attainments) and socioeconomic factors such as family background. However, if some country does not have standardized final high-school exams, the GPAs obtained can hardly be compared across different high schools. From that perspective, differences in high-school programmes (such as differences in gymnasium vs. Vocational programs) can lead to a student’s integration with higher educational systems being easier or more difficult. Overview of present studyThe aim of the present study was to further investigate predictors of AP in tertiary education using longitudinal research design.

AP was operationalized as a binary variable indicating whether the participant graduated at university level or not. While psychological correlates of AP are a widely-researched topic, we wanted to examine whether type of high school moderates the relationship of individual differences in intelligence, GK and personality to AP. Since the Croatian education system allows students from both gymnasium and vocational school to enter universities, the present study investigates whether individual differences or prior education contribute more to success at university.Intelligence and personality traits are constructs that represent qualitatively distinct individual differences, and it could be expected that there will be small or insignificant correlations between them ,.

However, some studies indicate that a small negative correlation between intelligence and Neuroticism might be found, due to mediational effects of test anxiety. Moderate correlation is expected to be found between intelligence and GK. Furthermore, it can be expected that intelligence and GK will be positive predictors of AP, while Psychoticism, Extraversion and Neuroticism will be negative predictors of AP. From the perspective of the Student Integration Model, we can expect that gymnasium leavers would perform better at university than those from vocational schools, merely due to the differences in primary goals between the two high-school programmes. A better scholastic background may lead to higher chances of better performance at higher educational levels.

MethodThis is an archival study, in which we have cross-referenced two archives of the Zadar Regional Office of the Croatian Employment Service (CES). Usage of the archive data was authorized by the Assistant Director of the CES. A psychologist, working as a career guidance counsellor, was in charge of data collection. No personal information about any participant was ever released outside the premises of the CES Zadar archives.The first registry used consists of the results of psychological assessment of high-school students. The CES offers a service of career guidance to all high-school students who are university candidates (leaving gymnasium or 4-year vocational school) and who seek advice in career choice. Students can voluntarily schedule a counselling session during their final high-school semester.

A psychologist gives career guidance, which consists of psychological assessment, semi-structured interview and counselling session. Typically, the psychological assessment consists of measures of intellectual abilities, general knowledge and personality. Four measures of intellectual abilities are used–the Problem Test (serving as a measure of reasoning ability) and three measures from the Multifactor Test Battery (serving as narrower measures of numeric, spatial and verbal abilities)–and a measure of general knowledge, while personality is assessed by Eysenck's Gigantic Three personality dimensions. Primarily, results of psychological assessment are used for counselling purposes; but they are also kept for a long-term psychometric evaluation of the counselling process.Information about AP was retrieved from the CES job-seeker database, the second registry used in this study. When an individual registers as a job seeker at CES, a counsellor collects their information about formal and informal education and work experience. If a participant who enrolled for career guidance during the years 2000–2005 could be found in the job-seeker database, information on their university graduation was collected (either that they graduated, or that they did not graduate).

If the information presented in the job-seeker database was inconsistent, or if there was no information for a certain participant, their test results were discarded.The dataset is available in the. Any personal information has been removed from the dataset, and only raw scores are presented. ParticipantsThe participants were final-year high-school students from Zadar County, Croatia, who enrolled in career guidance counselling at the CES, Zadar, between the years 2000 and 2005.

During that period, a total of 1389 students enrolled in counselling. If the participants’ data could be matched with the job-seeker registry, the data was recorded, and 826 participants (average age of 18.07 ( SD =.70)) were included in analyses.

A total of 239 (28.9%) were men, and 578 (71.1%) were women. Of these, 538 (65.1%) had enrolled in a gymnasium programme, while 291 (34.9%) had enrolled in a 4-year vocational school programme. Problem testThe Problem Test is a maximum-performance test that measures reasoning ability through problem-solving tasks.

It consists of 70 tasks (mostly verbal, and some numeric), and the participant is supposed to identify the problem that lies underneath the task, and report a solution for a given problem. Cronbach alpha internal reliability has been reported to lie between.85 and.95 in various studies. Problem identification and solving is considered a valid cognitive-ability measure. Multifactor test battery (MFTB)The MFTB is a Croatian adaptation of the General Aptitude Test Battery (GATB) , a test widely used for purposes of professional orientation and selection. In the current study, three sub-tests were used, and all of them were maximum-performance tests.

MFTB 2 is a numeric test, and the participant’s task is to perform simple maths operations as quickly as possible. It consists of 50 items. MFTB 3 is a spatial-representation test made up of 40 items, which are pictures of flat, two-dimensional objects with foldable edges. The participant’s task is to mentally ‘fold’ the planes of each object, and to select the correct three-dimensional object from among four choices.

MFTB 4 is a vocabulary test. It consists of 60 items, and each of them has one pair of distractors and one pair of either synonyms or antonyms. The participant’s task is to mark the two words that are either synonyms or antonyms.

Split-half reliability coefficients of the tests were:.92 (MFTB 2),.88 (MFTB 3) and.92 (MFTB 4). ResultsDescriptive statistics and bivariate correlations among intelligence measures (PT and three subtests of MFTB), personality measures (Eysenck's PEN personality model), GK, AP and type of high school are presented in. Prior to data analyses, assumptions for inferential statistical tests were examined. Indices of asymmetry (AI) and kurtosis (KI) indicated normal distribution of continuous variables (all AI. MSDSIKICorrelations2.3.4.5.6.7.8.9.10.1.PT40.6510.41.44.21.43.55.58.58.01.04.08.23.49.2.MFTB220.294.29.061.74.24.34.26.11.03.05.21.29.3.MFTB319.566.22.06.34.35.34.05.02.11.13.20.4.MFTB436.559.95.44.17.58.05.07.06.21.42.5.GK25.2111.97.05.72.04.13.06.24.51.6.P4.432.45.79.68.00.13.12.067.E15.393.77.99.73.24.13.018.N10.964.83.18.640.04.059.AP% (graduated)% (did not graduate).31.10.HS(g)47.817.3HS(v)14.520.4. Model 1Model 2Model 3Model 4BExp(B)pBExp(B)pBExp(B)pBExp(B)pHS1.353.96.00.972.62.001.002.72.001.002.73.00PT.001.00.97.001.00.89.011.00.78MFTB 2.061.06.00.061.06.01.101.11.00MFTB 3.011.01.62.011.01.38.02.98.49MFTB 4.001.00.66.001.00.82.021.02.20GK.021.02.07.021.02.09.011.01.46P.10.90.00.07.93.19E.07.93.00.04.96.31N.031.03.13.021.02.43HSxPT.011.01.69HSxMFTB 2.07.93.10HSxMFTB 3.051.06.09HSxMFTB 4.03.97.16HSxGK.011.01.64HSxP.06.94.37HsxE.06.95.25HSxN.011.01.86%68.169.169.970.2χ 279.59, df = 1, p.

HS- High school type; PT- Problem test; MFTB 2- Numeric test; MFTB 3- Spatial test; MFTB 4- Verbal test; GK- General knowledge; P- Psychoticism; E- Extraversion; N- Neuroticism;%- percentage of correct predictions; χ 2- Model chi-square, -2LL- -2 Log likelihood; B- unstandardized regression coefficient; exp(B)- The odds ratioIn Model 1, the contribution of the type of high school in predicting the AP was examined. It was found that high-school type was a significant predictor (χ 2 = 79.59, DF = 1, p.05). In the further discussion, we will interpret the results of Model 3. DiscussionIn this study, we examined the longitudinal contributions of intelligence, personality, GK and high-school type in predicting AP, operationalized as university graduation. The results presented in indicate that students from gymnasium high school (compared to those from vocational schools) have greater scores on all measures of intellectual abilities and in GK, while students from the different schools did not differ in personality traits.

Moreover, students who managed to graduate (compared to those who did not) had greater scores on all intellectual-ability measures, and in GK. In addition, they had lower levels of Psychoticism and Extraversion.

However, when all predictors of AP were introduced in the regression model (, Model 3) it was shown that only high-school type, numeric ability, Psychoticism and Extraversion were related to AP. Moreover, high-school type was shown to be the best predictor of graduation. Interpretation of exponentiated unstandardized b coefficients shows that gymnasium students have 172% greater chances of university graduation, that an increase of one point in the numeric-ability test leads to 6% higher chances of university graduation, and that an increase of one point on the Psychoticism and Extraversion scales leads to a lower probability of graduation by 11% and 7%, respectively.

Interaction terms between high-school type and all psychometric measures (Model 4, ) were insignificant. It seems that intelligence is a weak predictor of AP, and that gymnasium students do not profit from being ‘more intelligent’.

It also seems that what students learn in high school contributes more to their success at tertiary educational level than their individual differences in intelligence, GK and personality.Previous studies have also shown that intelligence is not a good predictor of AP at post-secondary educational level ,. In line with previous studies, introduction of intelligence measures led to a small increase (0.03) in pseudo-R 2 indices. For example, Kappe and van der Flier reported that intelligence accounted for 5% of the variance in students’ GPAs, while Farsides and Woodfield reported 4% of final grade being explained by intelligence.

The relation between intelligence and scholastic achievement is expected to be lower at university level than at lower levels of education , , because of the restricted range of the population at university level.Furthermore, it is interesting that GK was not a significant predictor of AP. Lack of this association might be due to the construction of TOIM, the GK measure used in this study. As already described in the Method section, TOIM is constructed to reflect knowledge different from that acquired in school. The lack of association between GK and AP in this study could mean that knowledge acquired in a general academic high-school programme, as offered by gymnasiums, could be far more relevant for performance at university.Moreover, the introduction of personality traits increased the predictive and incremental validity of AP prediction, which was in line with previous findings ,. Psychoticism was found to be negatively related to AP.

This is among the most consistent findings regarding the Gigantic Three personality traits and AP ,. Individuals high on Psychoticism often show behaviour such as poor cooperation with the group, have weak organizational skills, and exhibit low achievement motivation. It is also argued that Psychoticism can serve as a proxy measure for low conscientiousness , a Big Five trait that usually accounts for a substantial amount of variance in AP ,.Extraversion was also shown to be negatively related to AP, which is in line with previous findings ,. While pupils may benefit from a higher Extraversion level, it seems that this trait can obstruct performance in academic tasks. Extraverts might tend to spend more time socializing than studying, and could be more easily distracted from studying, thus having lower probability of university graduation.

The results did not reveal a significant relation between Neuroticism and AP in this study. In a number of studies, a negative relationship between Neuroticism and AP has been demonstrated ,. One possible explanation for the absence of this relation in the present study might be related to the method of measuring AP. AP is often measured through maximum-performance tasks (e.g. Test scores).

In that context, the negative emotionality that comes with high Neuroticism (e.g. Test anxiety) may negatively influence performance. However, university graduation is not a maximum-performance measure, and the effects of Neuroticism may be reduced when scholastic tasks are spread over several years. Neurotic students with high levels of motivation may demonstrate several behaviours for overcoming difficulties caused by anxiety: they can learn more, make an efficient plan for attending exams, or take more than one exam for the same course and improve their performance. Researchers have usually used a narrower operationalization of AP.

University graduation is a result of several (usually four or five) years of studying, during which students must accomplish various academic tasks and pass numerous exams. Many factors (both internal and external) in that process may influence one’s graduation and lower the correlation between intelligence and graduation.It should be noted that the Big Five personality dimensions might better predict differences in AP than Eysenck’s Gigantic Three model, but they could not be used in this study, since that data was not available in the present records. For a review of research on Big Five personality traits and AP, see ,. Two of those traits that could be particularly important in the prediction of AP are conscientiousness (representing students who are more motivated to perform well and more persistent when faced with difficulties ) and openness (representing students that are more imaginative, and might better manage new learning ). Therefore, it might be relevant for future study to explore the relationships of the Big Five traits to AP regarding students’ high-school background.Besides intelligence and personality measures, the results of this study revealed that high-school type is a significant predictor of AP.

Unsurprisingly, the results revealed that students from gymnasium high school, compared to vocational school, have a greater chance of graduating at university. It is worth noting that gymnasium students did not benefit from having greater intelligence levels than vocational-school students. (All interactions between high-school type and intelligence were non-significant.) This may imply that the high-school programme has a bigger impact on post-secondary education than do students’ abilities. In that respect, gymnasium schools fulfil their purpose of preparing students for university studies. As previously mentioned, intelligence might put a limit on what an individual can achieve, while personality might indicate how an individual will achieve it.

Here, we put emphasis on the context in which an individual is making his achievement. These findings are in line with Tinto’s Student Integration Model ,. Gymnasium leavers might be better prepared for university, since their high-school programme is designed for that purpose.

That might lead to better adjustment to the new study system, and greater goal commitment, leading to more persistence and, finally, greater achievement. On the other hand, vocational school programmes are divided between general education and skills that are required in the labour market. While vocational-school students can start work immediately after leaving, it seems that they might have difficulties in adapting to university studies. Although they are eligible for university admission, it seems that the current education system fails to prepare them sufficiently for post-secondary education.The results presented have two practical implications. First, the type of prior education can be a valuable source of information in future research on AP correlates.

It could be a simple measure of prior educational context in school systems similar to that in Croatia. Moreover, future studies should focus on more detailed explanation as to why gymnasium students perform better at university level. Other contextual factors that could be considered include parental support (Do more aspirant parents send their children to better schools and provide better support?), school social groupings (Do high-scoring students at the end of elementary school regress to different means in different high schools?) and university context (Do graduates from different high schools choose different universities with different standards for awarding degrees?).The second implication relates to equal possibilities for the higher education of those leaving different types of high school.

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The European Union is striving to increase inclusion in higher-education programmes. Although they are eligible university candidates, it seems that students from vocational schools in Croatia perform more poorly at university level. Therefore, they might benefit from some sort of additional institutional help prior to higher education. For instance, some countries (e.g. The Czech Republic, France and the Netherlands) offer additional counselling for high-school students. On the other hand, some countries offer additional preparation programmes (e.g.

The Czech Republic, Luxembourg and Iceland) prior to university entry. Limitations of the present studyThere are several limitations of this study that should be addressed. First, the generalizability of the data presented is limited due to the sample used in this study.

The participants in this study were final-year high-school students who decided to take career guidance counselling, and who, later in their lives, registered as job seekers with the CES. It should be noted that the entire sample showed a considerably high level of achievement motivation, which manifested itself in seeking career advice, and in actively searching for a job. However, although the sample size is reasonably large, the sample is not representative of the population of students.

There are final-year high-school students that did not engage in career guidance counselling (for it was not mandatory), and there are students that did engage in career guidance counselling, but later in life did not register as job seekers (which was not mandatory, either). Not engaging in career guidance can imply both high and low scholastic motivation. For example, an excellent student who achieves high grades and knows exactly which university to attend does not need career guidance. On the other hand, a student with low grades, and without any motivation to continue studies at university level, might find career guidance unattractive. Not registering as a job seeker with the CES can also imply both a successful career and lack of motivation to look for a job. Despite some limitations regarding the unrepresentative sample, the relationships between intelligence, personality and AP presented here are in line with the literature in this field, but these findings should be confirmed on a more representative sample.

The final regression model shows small pseudo-R 2 measures. It should be taken into account that this might be due to the operationalization of AP. A continuous measure of university graduation (e.g. GPA) might serve as a better dependent variable.

Moreover, on the basis of the available data, it was not possible to account for differences among universities and among different departments. It could be assumed that different departments have different standards that students must meet prior to graduation. This type of distinction at university level might provide better distinction of AP.

A number of exotic structures have been formed through high-pressure chemistry, but applications have been hindered by difficulties in recovering the high-pressure phase to ambient conditions (i.e., one atmosphere and 300 K). Here we use dispersion-corrected density functional theory PBE- ulg (Perdew-Burke-Ernzerhof flavor of DFT with the universal low gradient correction for long range London dispersion) to predict that above 60 gigapascal (GPa) the most stable form of N 2O (the laughing gas in its molecular form) is a one-dimensional polymer with an all-nitrogen backbone analogous to cis-polyacetylene in which alternate N are bonded (ionic covalent) to O. The analogous trans-polymer is only 0.03 ∼0.10 eV/molecular unit less stable. Upon relaxation to ambient conditions, both polymers relax below 14 GPa to the same stable nonplanar trans-polymer. The predicted phonon spectrum and dissociation kinetics validates the stability of this trans-poly-NNO at ambient conditions, which has potential applications as a type of conducting nonlinear optical polymer with all-nitrogen chains and as a high-energy oxidizer for rocket propulsion. This work illustrates in silico materials discovery particularly in the realm of extreme conditions (very high pressure or temperature).

With strong interplay between experiment and theory, such molecular crystals as N 2 (, ), CO 2 (, ), CO (, ), NH 3 , and benzene have been transformed into extended solids (covalent and ionic bonded networks) under high pressures. These studies have enhanced our understanding of chemical bonds under compression and provide opportunities to seek additional novel materials; however, it has been difficult to retain these remarkable structures at the ambient conditions needed for most applications. For CO 2, a 3D covalent network was synthesized at high pressure (40 GPa) and temperature (1,800 K) that is isomorphic to the β-cristobalite phase of SiO 2 , with each carbon atom bonded tetrahedrally to four oxygen atoms. This phase of CO 2 was proposed to have potential applications as superhard (initial experiments estimated a bulk modulus of 365 GPa , but theory and experiment later found it to be 136 GPa (, )), nonlinear optical, and high-energy density material, so efforts were made to quench this phase down to 1 atm and 300 K ; however, it reverts back to the molecular phase at pressures lower than 1 GPa.Because it is isoelectronic to CO 2 but polar, attempts were made to form an extended solid from N 2O using compression (above 20 GPa) and laser heating (above 1,000 K) in a diamond anvil. However, instead it decomposed into a mixture of an ionic crystal NO +NO 3 − and compressed N 2 molecules. No covalent extended framework similar to the polymeric CO 2 phase was found. Indeed, because the nitrogen atom forms one less covalent bond than the carbon atom, it is not obvious that it would be possible to construct a dense extended solid phase of N 2O other than the ionic form.

Even so, we decided to use first principle method to explore a large number of space groups and bonding patterns at high pressures.Because we start with molecular crystals of nitrous oxide (NNO) in which London dispersion attractions (van der Waals attraction) is dominant, we used the PBE- ulg flavor of density functional theory (DFT- ulg) in which corrections accounting for London dispersion attractions are included. We searched for high-pressure structures with two independent strategies.In the first approach, we started with 41 known AB 2-type crystal structures, replaced A with O and B with N, and used DFT to determine the optimum atom positions and packings using a fixed density of 3.915 gm/cm 3 (a relative compression of 0.384 and the same as polymeric CO 2 at 41 GPa). Then we took the new configurations from the lowest energy structures, built appropriate extended cells, and refined further with DFT to obtain the optimum stacking.In the second approach, we used the USPEX (Universal Structure Predictor: Evolutionary Xtallography) code (–) based on an evolutionary algorithm developed by Romanov, Glass, and Lyakhov and featuring local optimization, real-space representation, and flexible physically motivated variation operators. Here we considered a 12-atom unit cell at 40, 60, and 80 GPa.

( A) Ground state enthalpy as a function of pressure predicted for various phases of N 2O, with illustrations of crystal structures and chemical formulations of single chain for planar cis-polymer ( B), planar trans-polymer ( C), and nonplanar trans-polymer ( D).In the range of 10–55 GPa and 1,000–3,400 K, the diamond anvil experiments on NNO observed a mixture of the ionic NO +NO 3 − phase in equilibrium with an N 2 molecule phase, formed from the dissociation of N 2O at high pressure and temperature (, ). We also calculated these phases at the pressures considered here. However, the structure of NO +NO 3 − is not well-established experimentally: experiments reported the formation of monoclinic P2 1 /m phase ionic salt from the high-pressure (2 GPa) reaction of an N 2 and O 2 mixture , whereas the original experiments on dissociation of N 2O under laser heating in a diamond anvil found an orthorhombic phase related to the aragonite to form after cooling (, ). Therefore, for the DFT study, we constructed the NO +NO 3 − crystal by cell relaxation starting with the aragonite structure. This led to an orthorhombic Pna2 1 phase ( and ), which we found to be 0.03 eV per formula more stable than the monoclinic phase. We carried out DFT calculations of N 2 molecular crystals, starting with the known phases (α, γ, ɛ, ζ) at low temperature , and at each pressure, the energy calculated to be most stable was used in the enthalpy calculations.

Surprisingly, this combination of NO +NO 3 − and N 2 is more stable than the NNO molecular phases for all pressures down to 0 GPa. This is plausible,as the heat of formation of N 2O molecule is 82 kJ/mol (1 kcal = 4.18 kJ) above N 2 and O 2, which were shown to form NO +NO 3 − at 2 GPa using 10.2 keV synchrotron X-ray radiation.shows that the cis and trans 1D polymeric phases remain energetically close ( cis more stable by 0.03 ∼0.10 eV/molecular unit) and become more stable than both the molecular and the mixed ionic phases at pressures above 60 GPa.

Including zero point energy (ZPE) and entropy from phonons has little effect :. including ZPE to obtain the 0 K equation of state (EOS), we find that the critical pressure for the transition from the mixed ionic phase to the cis-polymer increases from 58.5 GPa to 61 GPa,. while introducing entropy and enthalpy corrections to 300 K leads to a further increase to 62 GPa at 300 K (64 GPa at 500 K).The dominant factor in determining the enthalpy at high pressure is the compressibility. For the mixed phase, the ionic NO +NO 3 − component is a dense solid, but the global compressibility of the mixed phase is dominated by the molecular phase of N 2, which polymerizes only above 110 GPa.

In contrast, the two new NNO polymeric phases each forms a pure extended solid with mixed covalent and ionic frameworks. Dissociation path calculated for dissociation of the model 8-mer. The oligomer contains 8 N 2O units and is terminated with methyl groups at both ends.After passing the TS, the oligomer would start releasing N 2O one by one from each end, accompanied with significant heat release of 40.6 kcal/mol per N 2O molecule, which might lead to catastrophic decomposition.

Thus, the nonplanar trans-NNO polymer is a high-energy content structural material. Indeed, we calculated (PBE- ulg) the nonplanar N 2O trans-polymer to provide an internal energy release of 3.5 kJ/g when dissociated into N 2 and O 2, which is comparable to the energy release of trinitrotoluene (TNT) (4.2 kJ/g). Thus, nonplanar trans-NNO is a potential high-energy oxidizer for new explosive composites and rocket propellants.Since the N 2O polymer chain is composed of alternating single and double bonds, similar to polyacetylene, albeit with an all nitrogen backbone, it might form the basis for a unique type of conducting polymer, through appropriate doping or structure modification. Similarly we expect that it might have strong nonlinear polarizabilities for nonlinear optical applications.Summarizing, we used DFT to predict that the NNO molecular crystal can be transformed into novel polymeric phases at high pressures (beyond decomposition into mixture phase of ionic compound NO +NO 3 − and N 2 gas previously observed in experiments). The two most stable 1D N 2O polymers with planar cis- and trans-conformations were identified to be energetically favorable at pressures above 60 GPa.

More importantly, when the pressure is released, these polymers transform into the same nonplanar trans-conformation, stable at ambient pressure and temperature. This was substantiated by analysis of the phonon spectrum and by calculating the dissociation kinetics. This unique poly-NNO material might be an excellent high-energy oxidizer for a polymer composite in which nonpolar NNO forms the matrix.The PBE calculations suggest metallic property in the high-pressure polymer resulting from the strong resonance in these systems, which seems to disappear in the low-pressure nonplanar polymer due to changes in bonding, leading to Peierls distortion, together with electrostatic repulsions. Thus, with doping these NNO polymers, such as I, which works successfully in polyacetylene, may give rise to a new type of conducting polymer based on all-nitrogen chains. This whole work serves to illustrate the unique in silico process of discovering materials by theoretical modeling particularly under extreme conditions.