Molecular Polarization Effects on the Relative Energies of the Real and Putative Crystal Structures of Valine

The computer-generation of the crystal structures of the α-amino acid valine is used as a challenging test of lattice energy modeling methods for crystal structure prediction of flexible polar organic molecules and, specifically, to examine the importance of molecular polarization on calculated relative energies. Total calculated crystal energies, which combine atom-atom model potential calculations of intermolecular interactions with density functional theory intramolecular energies, do not effectively distinguish the real (known) crystal structures from the rest of the low energy computer-generated alternatives when the molecular electrostatic models are derived from isolated molecule calculations. However, we find that introducing a simple model for the bulk crystalline environment when calculating the molecular energy and electron density distribution leads to important changes in relative total crystal energies and correctly distinguishes the observed crystal structures from the set of computer-generated possibilities. This study highlights the importance of polarization of the molecular charge distribution in crystal structure prediction calculations, especially for polar flexible molecules, and suggests a computationally inexpensive approach to include its effect in lattice energy calculations.