A polarizable electrostatic model of the N-methylacetamide dimer

Our previously developed polarizable electrostatic model is applied to isol ated N-methylacetamide (NMA) and to three hydrogen-bonded configurations of the NMA dimer. Two versions of the model are studied. In the first one (PO LI), polarizability along the valence bonds is described by induced bond ch arge increments, and polarizability perpendicular to the bonds is described by cylindrically isotropic induced atomic dipoles. In the other version (P OL2), the induced bond charge increments are replaced by induced atomic dip oles along the bonds. The parameterization is done by fitting to ab initio MP2/6-31++G(d,p) electric potentials. The polarizability parameters are det ermined by subjecting the NMA molecule to various external electric fields. POL1 turns out to be easier to optimize than POL2. Both models reproduce w ell the ab initio electric potentials, molecular dipole moments, and molecu lar polarizability tensors of the monomer and the dimers. Nonpolarizable mo dels are also investigated. The results show that polarization is very impo rtant for reproducing the electric potentials of the studied dimers, indica ting that this is also the case in hydrogen bonding between peptide groups in proteins. (C) 2001 John Wiley & Sons, Inc.