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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 (1979) 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
spectroscopic 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
cyrstals. 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).
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