THIS INFORMATION IS OBSOLETE AND IS PROVIDED ONLY FOR ITS HISTORICAL VALUE

by Donald E. Williams, Chemistry Department, University of Louisville, Louisville, Kentucky 40292

PCK83 calculates crystal lattice energies of molecular crystals and finds crystal structures with minimum energy. Usually, the molecule is considered to be rigid, but limited provisions are made for internal rotations about bonds. The general procedures used are presented in a paper published in Acta Crystallographica, A28, 629-635 (1972). A recent review appears in Topics in Current Physics, 26, 3-40 (1981). An earlier version of this program was called PCK6 and was listed as QCPE 373 (1979).

The intermolecular or nonbonded energy of the crystal is represented by a pairwise sum over atoms in different molecules. The program accepts either (exp- 6-1) or (n-6-1) nonbonded interatomic potentials (referred to as Buckingham or Lennard-Jones functions). A torsional potential is accepted for rotations about internal bonds. Provision is made for net atomic charges or lone-pair electron-site charges. The structural variables considered by the program are the six lattice constants, three molecular rotations, three molecular translations, and internal rotations. An external hydrostatic pressure on the crystal may be included.

Calculations can be made with the observed space-group symmetry or with no assumed symmetry. The energy and structure of molecular clusters can be calculated. There can be more than one independent molecule in the crystallographic asymmetric unit. Evaluation of the crystal lattice sums uses the accelerated convergence method (Acta Crystallographica, A27, 452 (1971)) so that high speed and accuracy can be attained. The first and second derivatives of the lattice energy are evaluated analytically, also using accelerated convergence. The program selects the Newton-Raphson method to find the calculated structure with minimum energy if the eigenvalues of the Hessian (second derivative) matrix are positive-definite; otherwise, the steepest descent method is used.

FORTRAN 77 (VAX 11/750) Lines of Code: 3215