Intermolecular Potential, Vibrational Spectra, and Structures of Amino Acid Crystals. I. Distortion of Molecules and Equilibrium Crystal Structure

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Abstract
A general method of the crystal packing optimization is developed for the molecular crystals consisting of non-rigid molecules. The intramolecular potential is described as a general harmonic force field in terms of a suitable set of internal symmetry coordinates, and the intermolecular potential as a sum of the pairwise atom-atom interaction terms. The adjustable structure parameters are those among the six cell constants, a, b, c, α, β, and γ, and the translations and the rotations of the molecules which maintain the crystal symmetry. The intermolecular force is separated into two parts, one of which determines the location of the center of mass and the orientation of the principal axes of each molecule and the other leads to a certain molecular distortion. The calculated distortion may be used, in addition to the difference between the calculated and the observed crystal structures, as a check point of the reliability of the model potential. The importance of this separation in the rigid molecule approximation is pointed out.