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Ab initio study of urea crystal

Xue D., Ratajczak H.

Bulletin of the Polish Academy of Sciences. Chemistry

2003

Vol. 51, No. 3-4

143-154

Dielectric properties - linear (refractive indices) and nonlinear optical responses - of urea crystal are quantitatively calculated using static ab initio quantum chemistry calculations at the self-consistent field (SCF) level of theory. The polarizabili-ties and first hyperpolarizabilities of the urea molecule are used to calculate macroscopic susceptibilities of the urea crystal. The calculated values of the urea crystal can agree well with experimental data and other reports. The effect of hydrogen bonds on the susceptibilities of the urea crystal may be neglected. The present work provides us a useful way to deal with quantum chemistry calculations of such molecular crystals as urea.

Phase transformations in locally anharmonic systems. Susceptibility approach to orientational instabilities in molecular solids

Kuchta B., Luty T.

Polish Journal of Chemistry

1998

Vol. 72, nr 2

426-432

A model of the structural transformation has been discussed, where the local potential is anharmonic but single-welled. Such potential representants a Hamiltonian to model the structural phase transition in charge-transfer molecular crystals A-TCNB (anthracene-tetracyanobenzene). The conditions for the instability and transformation have been found, using the static susceptibility approach, and compared with the variational approximation. The latter method gives a six times lower transition temperature. The fluctuations and entropy are pointed out as the cause of this difference. The exact transition temperature has been calculated by simple Monte Carlo simulations. A comparison with a double-welled case, which models the structural transition in N-TCNB molecular crystal (naphthalene-tetracyanobenzene), has been presented.