ON THE SINGLET-TRIPLET SEPARATION IN METHYLENE - A CRITICAL COMPARISON OF SINGLE-VERSUS 2-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. © 1995 American Institute of Physics.