Fragment-Molecular-Orbital-Method-Based ab Initio NMR Chemical-Shift Calculations for Large Molecular Systems

An ab initio computational method, based on the fragment molecular orbital (FMO) method, for calculating nuclear magnetic resonance (NMR) chemica shifts has been developed by introducing the concept of a merged fragment with a cutoff distance. Using point charges or density based on electrostatic potential obtained from FMO calculations, the NMR calculations (CIAO and CSGT) with the 6-31G(d) and 6-311G(d,p) basis sets were performed on cc-helix and beta-sheet polypeptides. The cutoff distance defines the optimal merged fragment size for NMR calculations. This method accurately reproduces electrostatic effects and magnetic susceptibilities. The chemical shifts determined with a cutoff distance not shorter than 8 angstrom for both alpha-helix and beta-sheet polypeptides agree well with those calculated by conventional ab initio NMR calculations.