optimum strategy for solution chemistry using semiempirical molecular orbital method. II. Primary importance of reproducing electrostatic interaction in the QM/MM framework

For the purpose of executing direct dynamic and statistical calculation of chemical reactions in solution, we proposed an optimum strategy using semiempirical molecular orbital (MO) method with neglect of diatomic differential overlap (NDDO) approximation with specific solution reaction parameters (SSRPs), that is, the NDDO-SSRP method. It has been further extended to develop the NDDO-MAIS-SSRP method, which is the NDDO-SSRP method reinforced with the method adopted for intermolecular studies (MAIS), to correct the description of the intermolecular core-core repulsion interaction energy. In this strategy, the empirical parameters of the semiempirical MO method are optimized individually for a target chemically reacting molecular system by reference to the ab initio MO calculation data for a lot of instantaneous geometries on the potential energy surface near the reaction path. For demonstration, the NDDO-MAIS-SSRP method was applied, within the QM/MM framework, to a molecular cluster, that is, a couple of a QM solute NH₃---H₂O molecule pair and a MM solvent H₂O molecule. The NDDO-MAIS-SSRP method can reproduce the electrostatic energy in the region R > 4.0 Å, though the electrostatic energy shows large difference with those of MP2 level calculations in the region R < 4.0 Å in some cases. Both the NDDO-SSRP and the NDDO-MAIS-SSRP methods could promise in the dynamical application to chemical reaction in solution (Takenaka et al., Chem Phys Lett 2010, 485, 119; Koyano et al., Bull Chem Soc Jpn, in press).