Accurate Double Many-Body Expansion Potential Energy Surface for the Lowest Singlet State of Methylene

A single-sheeted double many-body expansion potential energy surface is reported for ground-state CH2 by fitting accurate ab initio energies that have been semiempirically corrected by the double many-body-expansion scaled-external-correlation method. The energies of about 2500 geometries have been calculated using the multireference configuration interaction method, with the single diffusely corrected aug-cc-pVQZ basis set of Dunning and the full valence complete active space wave function as reference. The topographical features of the novel global potential energy surface are examined and compared with other potential energy surfaces.