Excited state direct dynamics of benzene with reparameterized multi-reference semiempirical configuration interaction methods

We investigate the feasibility of reparameterized multi-reference semiempirical methods in the context of direct wavepacket dynamics, where the electronic and nuclear Schrodinger equations are solved simultaneously. Using a very limited set of ab initio and experimental data, we refine the semiempirical parameters for benzene. The resulting reparameterized semiempirical method is compared to ab initio results including dynamic electron correlation over selected cuts of the ground and excited state PESs. We demonstrate that reparameterization with a limited data set can provide globally correct PESs. We also present the results of minimal energy conical intersection optimizations for benzene using correlated wavefunctions (CASPT2). Finally, we simulate the dynamics of benzene after excitation to S-2 using both the standard and newly optimized parameter sets. We show that the excited state lifetime using the standard parameter set is inconsistent with experiment. In contrast, the newly optimized parameter set predicts an S-2 lifetime of less than 100 fs, in agreement with experiment. The reparameterization strategy is a promising way to extend direct dynamics simulations of photochemistry to multi-picosecond time scales. (C) 2004 Elsevier B.V. All rights reserved.