A polarizable mixed Hamiltonian model of electronic structure for micro-solvated excited states.: I.: Energy and gradients formulation and application to formaldehyde (1A2)

We describe an efficient implementation of a polarizable mixed Hamiltonian model of electronic structure that combines Hartree-Fock, Kohn-Sham, or multiconfiguration quantum-chemical wave functions with a polarizable and flexible molecular mechanics potential of water, and that is applicable to micro-solvated electronic excited states. We adopt a direct algorithm for the calculation of the polarization response of the solvent subsystem. The strategy facilitates the calculation of the energy of the system and of the forces with respect to the solute coordinates and the solvent coordinates, including for excited states. This capability opens the way to the determination of optimized, transition structures, force constants, and intrinsic reaction pathways for the solute-solvent system, and to molecular dynamics calculations to account for finite temperature effects. As an illustration we characterize the structure and energy of micro-solvated formaldehyde H2CO in its ground state and in its (1)(pi*<--n) excited state. A novel perpendicular structure is found to be the lowest energy conformation of the H2CO1(pi*<--n):H2O complex. The all-quantum-chemical results and the mixed Hamiltonian results, with or without solvent polarizability, are in semiquantitative agreement. We comment on the choice of Lennard-Jones parameters associated with a solute excited state. Lennard-Jones parameters that yield good ground state structures and energies with the mixed Hamiltonian model, are found to be too soft for the micro-solvated excited state H2CO in the adiabatic (equilibrium micro-solvation) regime. (C) 2002 American Institute of Physics.