A new (multi-reference configuration interaction) potential energy surface for H2CO and preliminary studies of roaming

We report a new global potential energy surface (PES) for H-2 CO, based on precise fitting of roughly 67 000 MRCI/cc-pVTZ energies. This PES describes the global minimum, the cis-and trans-HCOH isomers, and barriers relevant to isomerization, formation of the molecular (H-2+CO) and radical (H+HCO) products, and the loose so-called roaming transition-state saddle point. The key features of the PES are reviewed and compared with a previous PES, denoted by PES04, based on five local fits that are 'stitched' together by switching functions (Zhang et al. 2004 J. Phys. Chem. A 108, 8980-8986 (doi: 10.1021/ jp048339l)). Preliminary quasi-classical trajectory calculations are performed at the total energy of 36 233 cm(-1) (103 kcal mol(-1)), relative to the H2CO global minimum, using the new PES, with a particular focus on roaming dynamics. When compared with the results from PES04, the new PES findings show similar rotational distributions, somewhat more roaming and substantially higher H-2 vibrational excitation. This article is part of the themed issue 'Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces'.