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High-pressure study of adamantane: variable shape simulations up to 26 GPa.
MedLine Citation:
PMID:  16853208     Owner:  NLM     Status:  PubMed-not-MEDLINE    
Abstract/OtherAbstract:
We report simulations of adamantane by carefully combining ab initio and empirical approaches to enable simulations with internal degrees of freedom on crystalline adamantane up to a pressure of 26 GPa. Two sets of simulations, assuming the adamantane molecule as a rigid (RB) and flexible body (FB), have been carried out as a function of pressure up to 26 GPa to understand changes in the crystal structure as well as molecular structure. The flexible body simulations have been performed by including 6 lowest frequency internal modes (obtained from DFT calculations performed with Gaussian98) out of the total of 72. The calculated variation in c/a and V/V(0) from the RB and FB calculations as a function of pressure have been compared with the experimental curve. Other relevant thermodynamic and structural properties reported are the radial distribution functions, deviation in the position of a given type of atom with respect to its position at standard pressure, delta(s), cell parameters, volume, and energy. With an increase in pressure, three additional peaks are seen to develop gradually at three different pressures in the center of mass (com)-com radial distribution function (rdf). We attribute these changes to structural transformations (probably second-order phase transitions) which is consistent with the three phase transitions reported by Vijayakumar et al. for adamantane in the pressure range of 1 atm-15 GPa. Our simulations also show that these additional peaks in the rdf's are associated with the differences between opposite and parallel spin neighbors of Greig and Pawley as well as the crystallographic directional dependence of intermolecular distances in the first three shells of the neighbors. Also, the structural quantities from the RB calculation show considerable deviation from the FB calculation for pressures greater than 5 GPa, which suggests that the rigid body assumption for molecules may not be valid above this pressure.
Authors:
N Arul Murugan; R S Rao; S Yashonath; S Ramasesha; B K Godwal
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Publication Detail:
Type:  Journal Article    
Journal Detail:
Title:  The journal of physical chemistry. B     Volume:  109     ISSN:  1520-6106     ISO Abbreviation:  J Phys Chem B     Publication Date:  2005 Sep 
Date Detail:
Created Date:  2006-07-20     Completed Date:  2007-06-26     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  101157530     Medline TA:  J Phys Chem B     Country:  United States    
Other Details:
Languages:  eng     Pagination:  17296-303     Citation Subset:  -    
Affiliation:
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India.
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