On the singlet–triplet separation in methylene: A critical comparison of single- versus two-determinant (generalized valence bond) coupled cluster theory

We present a critical comparison of the performance of the single- vs two-determinant-reference coupled cluster method, including its generalized valence bond version, for the classic multireference problem of the singlet–triplet separation in methylene. After demonstrating excellent agreement between the two-determinant coupled cluster method with single and double excitations and the full configuration interaction method for a double zeta polarization (DZP) basis, we adopt an extended atomic natural orbital basis and obtain harmonic frequencies for the two states to give T0=10.30 kcal mol−1 and 8.86 kcal mol−1, respectively, for the single- and two- determinant-reference coupled cluster results, compared to the experimental value, 8.998±0.014 kcal mol−1. Adding triples, the corresponding single-reference value is 9.35 kcal mol−1. We also consider stretched geometries of CH2 as a stringent test of our approach. Comparisons are made between the two-determinant coupled cluster values, including the single and double excitations and their triples excitation corrected coupled cluster counterparts, obtained using various choices of orbitals, including the generalized valence bond ones.