Ab initio molecular dynamics studies of the photodissociation of formaldehyde, H2CO -> H-2+CO: Direct classical trajectory calculations by MP2 and density functional theory

The dynamics of H2CO -->H-2+CO photodissociation have been studied by class ical trajectory calculations at the MP2/6-311G(d,p), B3LYP/6-311G(d,p), and VSXC/6-311G(d,p) levels of theory. The trajectories were calculated direct ly from the electronic structure computations without first fitting a globa l potential energy surface. A Hessian based method with updating was used t o integrate the trajectories. The translational energy distribution of the products is in better agreement with experiment than the previous Hartree-F ock direct trajectory calculations, since the MP2 and density functional me thods reproduce the barrier height better. The MP2 and density functional c alculations give very good descriptions of the product rotational state dis tributions and the CO vibrational state populations. The MP2 method yields a very good representation of the H-2 vibrational state populations, wherea s the density functional methods give too little H-2 vibrational excitation and Hartree-Fock produces too much. This can be attributed to the differen ce in the potential energy release that accompanies the formation of the H- 2 bond. (C) 2000 American Institute of Physics. [S0021-9606(00)01646-9].