Ab initio molecular dynamics studies of the photodissociation of formaldehyde, H2CO→H2+CO: Direct classical trajectory calculations by MP2 and density functional theory

The dynamics of H2CO→H2+CO photodissociation have been studied by classical 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 directly from the electronic structure computations without first fitting a global potential energy surface. A Hessian based method with updating was used to integrate the trajectories. The translational energy distribution of the products is in better agreement with experiment than the previous Hartree–Fock direct trajectory calculations, since the MP2 and density functional methods reproduce the barrier height better. The MP2 and density functional calculations give very good descriptions of the product rotational state distributions and the CO vibrational state populations. The MP2 method yields a very good representation of the H2 vibrational state populations, whereas the density functional methods give too little H2 vibrational excitation and Hartree–Fock produces too much. This can be attributed to the difference in the potential energy release that accompanies the formation of the H2 bond.