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Manuscript Title: Simulation of molecular reorientation in crystals.
Authors: J.C.A. Boeyens, D.C. Levendis
Program title: ORIENT
Catalogue identifier: AADZ_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 39(1986)221
Programming language: Fortran.
Computer: CDC CYBER 750.
Operating system: CDC NOS. 2.2.
RAM: 62K words
Word size: 60
Peripherals: magnetic tape.
Keywords: Crystallography, Molecular reorientation, Lattice energy, Concerted rotation, Disorder, Phase transition, Simulation structure.
Classification: 8.

Nature of problem:
Phase transitions of the lambda-type are common in molecular crystals where disorder-order transformations are possible because of molecular reorientation. Simulation in terms of the total interaction of a rotating molecule with the whole lattice is often inconclusive and the physical process would be better simulated by reorientation in a non- static environment.

Solution method:
Reorientation of disc-like molecules in crystals are simulated by rotation about molecular plane normals with preservation of translational symmetry. To allow for the independent reorientation of symmetry-related molecules all simulations are done in space group P1. The lattice potential energy over a suitable fragment of interest is calculated at each rotational state and mapped in rotational space.

The program handles up to three independent molecular or rigid-group rotations about any specified vectors. In all successful applications to date however, the problems were defined in terms of various sets of two independent rotations about the plane normals of flat molecules. Potential energy is calculated at rotational stations only and minimization as a function of individual atomic coordinates is not provided for. Automatic adjustment of cell parameters cannot be done.

Unusual features:
Energy minimization routines cannot escape from local minima. This program can serve to locate sub-minima in potential energy surfaces, to be followed by minimization.

Running time:
For 13 step rotations of two molecules of 52 atoms each and a cut-off at 5 Angstrom, 470 seconds of CDC 750 CPU time is required.