Octahedral point-charge model and its application to fragment molecular orbital calculations of chemical shifts

To obtain chemical shifts in large molecular systems accurately with a low computational cost, we developed an octahedral point-charge model that mimics the electrostatic potential due to charge distributions. The point-charges in this model are defined to reproduce the multipole moments calculated using the revised distributed multipole analysis. The accuracy of charge representations was tested on formamide. The octahedral point-charge model was used in the fragment molecular orbital method and applied to NMR calculation of a β-sheet polypeptide. The maximum errors relative to conventional ab initio NMR calculation were 0.13, 0.73, and 0.09 ppm for 13C, 15N, and 1H, respectively.