Structures and vibrational frequencies in the full configuration interaction limit: Predictions for four electronic states of methylene using a triple-zeta plus double polarization (TZ2P) basis

Benchmark energies, geometries, dipole moments, and harmonic vibrational frequencies are determined for four states of methylene (CH2) by solving the electronic Schrödinger equation exactly for a triple-ζ plus double polarization (TZ2P) basis with the restrictions that the core orbital remains doubly occupied and the highest-lying virtual orbital is deleted. Approximate models of electron correlation are evaluated based on their ability to match the exact, full configuration interaction results. Predictions from configuration interaction with all single, double, triple, and quadruple substitutions (CISDTQ) and coupled-cluster including singles, doubles, and triples (CCSDT) are virtually identical to the full CI results for all but the c̃ 1A1 state, which is poorly described by a single-configuration reference. In agreement with previous work, the c̃ state remains slightly bent at the TZ2P full CI level of theory, with a bond angle of 170.1° and a barrier to linearity of only 25 cm−1.