Accurately solving the electronic Schrodinger equation of small atoms and molecules using explicitly correlated (r12-)MR-CI. VIII. Valence excited states of methylene (CH2).

We compute the adiabatic transition energies of methylene (CH(2)) from the ground state to the lowest electronically excited valence states using the r(12)-MR-ACPF-2 method with a large basis set and an extended reference space. We recall that this method aims at reaching the basis-set and full configuration interaction (CI) limits simultaneously. Our best excitation energies, T(e) (T(0)), are 9.22 (8.87) (a (1)A(1), corrected for relativistic and adiabatic effects), 31.98 (31.86) (b (1)B(1)), and 57.62 (57.18) kcal mol(-1) (c (1)A(1)) (both uncorrected). We are able to reach the respective basis-set limits that closely that the remaining errors of our (uncorrected) calculations are clearly due to the MR-ACPF-2 method. While we are unable to assess the error of the latter method in a systematic way, we still believe that it is rather unlikely that the errors of our excitation energies exceed +/-0.10 kcal mol(-1). We finally observe that our (corrected) a state values deviate by only -0.10 (-0.10) kcal mol(-1) from the results of Csaszar et al. [J. Chem. Phys. 118, 10631 (2003)]--who did careful extrapolations to the valence full-CI and basis-set limits and added a correction for the core correlation--and that the deviation from experiment is only -0.13 (-0.13) kcal mol(-1). From these excellent agreements we conclude that our excitation energies to the b and c states are similarly accurate.