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325. MCA: A General Program for Molecular Crystals
Analysis
by E. Huler, R. Sharon, and A. Warshel, Department of Chemical Physics, Weizmann Institute of Science, Rehovot, Israel This program performs calculations of the type described by A. Warshel and S. Lifson, J. Chem. Phys., 53, 582 (1970) and by E. Huler and A. Warshel, Acta Cryst., B30, 1822 (1974). It is an extension of the QCFF/PI program (QCPE 247) to calculations of crystal energies, equilibrium geometries, and lattice vibrations of molecular crystals. In contrast to the previously used rigid body approximations, MCA allows for a simultaneous calculation of the effect of inter- and intramolecular forces on crystal packing and lattice dynamics. The crystal potential is given by the quantum mechanical extension of the consistent force field to p-electron systems (QCFF/PI). This potential is analytically represented as a function of the 3n Cartesian coordinates of the n atoms in the asymmetric unit and the 6 unit cell parameters. The method provides analytical expressions for the first and second derivatives of the potential which are utilized in the calculations of both equilibrium geometries and lattice vibrations. The effect of crystal forces on the structure of flexible molecules can be evaluated. The calculated lattice vibrations can be used for spectroscopical studies, for direct evaluation of thermal expansion, and for the evaluation of the atomic thermal ellipsoids. The method can also be used for studying electronic excited states of pure and mixed molecular crystals. Thus, it is possible to evaluate the conformational changes upon electronic excitations. Such calculations allow the study of excimer formation and photochemical reactions in molecular crystals (see, e.g., A. Warshel and E. Huler, Chem. Phys., 6, 463 (1974)). FORTRAN IV (IBM 370) Lines of Code: 6953 Recommended Citation: A. Warshel et al., QCPE 11, 325 (1977). |