Crystal structure prediction by global optimization as a tool for evaluating potentials: Role of the dipole moment correction term in successful predictions

A recently proposed method for surmounting the multiple-minima problem in protein folding is applied here to the prediction of crystal structures by global optimization of a potential energy function. The method, self-consistent basin-to-deformed-basin mapping, locates a group of large basins (regions of attraction of single minima) containing low-energy minima in the original energy surface, by coupling these groups of minima in the original surface to basins in a highly deformed energy surface, which contains a significantly reduced number of minima. The experimental crystal structures of formamide, imidazole, and maleic and succinic anhydrides were predicted as the global minima of the AMBER potential and were found among the lowest-energy minima for the DISCOVER potential. The results of the predictions serve as tests for evaluating the two potentials and may serve as a guide for potential refinements. Another important goal of this study was to clarify the role of the dipole moment contribution in calculations of the crystal electrostatic energy when the dipole moment of the unit cell is nonzero. Contrary to some practices, it is suggested that the use of the Ewald summation formula alone, without correcting for the dipole moment of the unit cell, is not the proper way to compute the electrostatic energy of a crystal and may lead to wrong predictions.