An ab initio molecular orbital-valence bond (MOVB) method for simulating chemical reactions in solution

A mixed molecular orbital and valence bond (MOVB) method for describing the potential energy surface of reactive systems has been developed and applie d to a model proton transfer reaction in aqueous solution. The MOVE method is based on a block-localized wave function (BLW) approach for defining the diabatic electronic states. Then, a configuration interaction Hamiltonian is constructed using these diabetic stales as the basis function. It was fo und that the electronic coupling energy is large with a value of about 30 k cal/mol for the H3N-H-NH3+ system, whereas the predicted activation barrier is only 1.2 kcal/mol using the 3-21 G basis set. The MOVE results are foun d to be in good accord with the corresponding ab initio Hartree-Fock calcul ations for the proton transfer process. We have also incorporated solvent e ffects into the MOVE Hamiltonian in the spirit of combined QM/MM calculatio ns. and have modeled the proton transfer between ammonium ion and ammonia i n water using Monte Carlo simulations. The potential of mean Force was comp uted via free energy perturbation coupled with umbrella sampling techniques using (1) an energy gap mapping approach, and (2) a geometrical mapping pr ocedure, Solvent effects: increase the barrier height by about 2.2 kcal/mol from the MOVE and HF ground stale potential energy surface. The present st udy demonstrated the feasibility of ab initio MOVE method for studying chem ical reactions by incorporating explicit solvent effects in the description of the reaction coordinate in combined QM/MM simulations.