Theoretical study of the low-lying excited singlet states of furan

2 (V), 1 A 1 (V8), respectively, at the C 2v ground-state molecular configuration# have been studied using the equation-of-motion coupled-cluster singles and doubles method ~EOM-CCSD!. Full geometry optimizations with subsequent computation of harmonic vibrational frequencies have been performed in order to locate and characterize stationary points on the potential energy surfaces ~PES!. The latter optimization work was enabled by the availability of analytic energy gradient techniques for the EOM-CCSD approach. A major new finding is that both the 1 B2(V) and 1 A1(V8) valence states are unstable with respect to non-totally symmetric distortions at the C2v configuration. The symmetry breaking in the 1 B2(V) state involves an in-plane coordinate of b2 symmetry. The relaxation process begins on the S2 adiabatic PES and, after passing through a conical intersection of the S2 and S1 PES, continues on the S1 surface, taking the system finally to the adiabatic minimum ofS1 ( 1 A2 state!. The 1 A1(V8) valence state is found to be unstable with respect to the out-of-plane bending coordinates of b1 and a2 symmetry. The resulting relaxed molecular structures have Cs and C2 symmetry, respectively. The present findings are analyzed in terms of a linear vibronic coupling model and spectroscopic implications are discussed. © 2003 American Institute of Physics. @DOI: 10.1063/1.1578051#