Semiclassical simulation of photochemical reactions in condensed phase

We compare two strategies for the semiclassical simulation of photochemical reactions in condensed phase (solutes in liquids, impurities in solid matrices, adsorbates, or photoreactive units in biological environments). Both strategies are based on classical nuclear trajectories, with surface hopping to simulate nonadiabatic transitions. In the first approach we calculate electronic energies and couplings by ab initio methods for many nuclear geometries of the reactive system (the 'solute'), and we fit the results by analytic functions of the internal coordinates. In the case of surface crossings it is mandatory to resort to a (quasi-)diabatic representation of the electronic states. A condensed state environment (the 'solvent') is described by ordinary molecular mechanics (MM), and the solute-solvent interactions can be state-specific. The other strategy is direct, i.e. it involves the calculation of electronic energies and wavefunctions at each integration step of a nuclear trajectory. The method we have implemented is based on semiempirical configuration interaction calculations with floating occupation SCIF orbitals; within this approach we have developed a hybrid quantum mechanics/molecular mechanics (QM/MM) procedure to represent the solvent, which is here briefly described for the first time. While the intra- and intermolecular potentials for the solvent molecules are of AM type, the QM/MM interaction is introduced in the semiempirical electronic hamiltonian, so that it influences in a state-specific way electronic energies and wavefunctions. Applications of both approaches to photoreaction dynamics are briefly described. (C) 2002 Elsevier Science B.V. All rights reserved.