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 p rotein folding is applied here to the prediction of crystal structures by g lobal optimization of a potential energy function. The method, self-consist ent basin-to-deformed-basin mapping, locates a group of large basins (regio ns of attraction of single minima) containing low-energy minima in the orig inal energy surface, by coupling these groups of minima in the original sur face to basins in a highly deformed energy surface, which contains a signif icantly reduced number of minima. The experimental crystal structures of fo rmamide, imidazole, and maleic and succinic anhydrides were predicted as th e global minima of the AMBER potential and were found among the lowest-ener gy 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 pot ential refinements. Another important goal of this study was to clarify the role of the dipole moment contribution in calculations of the crystal elec trostatic energy when the dipole moment of the unit cell is nonzero. Contra ry to some practices, it is suggested that the use of the Ewald summation f ormula 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 ma y lead to wrong predictions.