Comparison of Static and Fluctuating Charge Models for Force-Field Methods Applied to Organic Crystals

We investigated different charge models in a force-field approach for calculating structural data of organic crystals. Charges were calculated according to the Gasteiger and charge equilibration (QEq) methods or were derived from the quantum-mechanical electrostatic potential (ESP). A fluctuating charge model, based on the QEq approach, for describing polarization effects was developed. Different variants of the fluctuating charge model as well as static charges were applied to a test set of 48 organic crystals using the Dreiding force field. On average, the inclusion of crystal field effects with the fluctuating charge model lead to slight improvements over static ESP charges, both in terms of the average error-with respect to experiment-and in terms of the standard deviation. The empirical charge models, Gasteiger and QEq, tend to be less accurate if compared with quantum-mechanically derived charges.