PROGRAM HARDPACK: PREDICTION AND OPTIMIZATION OF CRYSTAL STRUCTURES USING ATOM-ATOM POTENTIALS AND POINT CHARGES. Rainer Rudert, Federal Institute for Materials Research and Testing, Rudower Chaussee 5, D-12489 Berlin (Germany), phone: (030) 6392 5858, fax: (030) 6392 5787, e-mail: rudert@chemie.fu-berlin.de or nwfrieden@gn.apc.org

A computer program is presented which is able to predict polymorphs of organic crystal structures. The program needs a 3-dimensional model of the molecule including the correct bond lengths, bond angles, and most of the torsional angles. The user may also supply atomic charges. The program varies cell constants, positions of molecules, orientations of molecules and selected torsional angles and calculates energies for each set of parameter values. There are several algorithms available to optimize the crystals structure with respect to the energy, including simulated annealing, grid search, steepest descent, Newton/Raphson, and Rosenbrock method.

The potential between two atoms i and j with distance r_ij is V(r_ij) = A_ijxexp(-B_ijxr_ij)/r_ij^Dij - C_ij/r_ij⁶ + q_ixq_j/r_ij, where A_ij, B_ij, C_ij, and D_ij are potential parameters depending on the atoms types, q_i and q_j are the atomic charges of atoms i and j. The user may also choose a "hard sphere" atomic model with V(r_ij > 1.1xr_vdw) = 0; V(r_vdw< r_ij < 1.1xr_vdw) = 1 kJ/mol; V(r_ij < r_vdw) = 10¹⁰ kJ/mol, r_vdw is the van der Waals radius. It is possible to optimize Langmuir-Blodgett Monolayers with a flat surface substrate and an atom-surface potential V(d_i) = E_i/d_i⁹ - F_i/d_i³, where d_i is the distance atom-surface and E_i and F_i are material constants.

Constraints between parameters may be defined. The program is controlled by a user written input file, which has a SHELX-style format.

The program is written in FORTRAN77 and has been tested under MSDOS, UNIX, and VAX/VMS.