On the triplet-singlet energy gap of acetylene

The triplet-singlet energy gap of acetylene, T-e(B-3(2)-(1)Sigma(g)(+)), wa s calculated using the coupler-cluster theory including all single and doub le excitations plus perturbative corrections for the triples, and multiconf igurational second-order perturbation theory methods with large basis sets. The cis-bent triplet state ((a) over tilde B-3(2)) is calculated to lie 30 500 +/- 500 cm(-1) above the ground singlet state ((1)Sigma(g)(+)); the la tter value differs somewhat from the most recent evaluation of T-0=28 900 c m(-1) [Amed , J. Chem. Phys. 110, 4248 (1999)] but agrees well with an earl ier theoretical estimate of 30 270 cm(-1) [Yamaguchi , Theor. Chim. Acta 86 , 97 (1993)]. Thus the discrepancy of 1000 cm(-1) may well arise from an in terpretation of experimental results rather than a shortcoming of theoretic al calculations. (C) 2000 American Institute of Physics. [S0021-9606(00)306 16-X].