Ab initio QM/MM simulations with a molecular orbital-valence bond (MOVB) method: Application to an S(N)2 reaction in water

A mixed molecular orbital and valence bond (MOVB) method isd described in c ombined nb initio QM/MM simulations of the S(N)2 reaction of Cl- + CH3Cl -- > ClCH3 + Cl- in water. The method is based on the construction of individu al charge-localized, diabatic states using a block-localized wave function approach, followed by configuration interaction calculations to obtain the adiabatic potential energy surface. To examine the performance of the MOVB method, modern ab initio VB calculations were performed. The MOVB gas phase results are found to be in reasonable agreement in the overall potential e nergy surface in comparison with Hartree-Fock, MP2, and ab initio VB calcul ations. The only exception is that the activation energy is predicted to be about 4 kcal/mol higher in MOVB than in other methods. This is attributed to the configuration interaction procedure, which does not further optimize orbital coefficients in MOVB calculations, and it emphasizes the importanc e of orbital optimization in these calculations. The adiabatic ground-state potential surface can also be approximate by other quantum chemical models in Monte Carlo QM/MM simulations. At the HF/6-31G(d) level, the calculated activation free energy of 26 kcal/mol in water is in good agreement with e xperiment and with previous computational results, importantly the MOVB met hod allows for the solvent reaction coordinate to be used to define the rea ction path in ab initio QM/MM simulations. (C) 2000 John Wiley & Sons, Inc.