Geometry optimization of excited valence states of formaldehyde using analytical multireference configuration interaction singles and doubles and multireference averaged quadratic coupled-cluster gradients, and the conical intersection formed by the 1 1B1(σ-π*) and 2 1A1(π-π*) states

Extended MR-CISD (multireference configuration interaction singles and doubles), MR-CISD+Q (multireference configuration interaction singles, doubles, and quadrupole), and MR-AQCC (multireference averaged quadratic coupled clusters) calculations have been performed on the following valence states of formaldehyde: 1 (1)A(1) (planar ground state), 1 (1)A(2) and 1 (1)A(') (planar and nonplanar n-pi* state), 1 B-1(1) and 2 (1)A(1) (planar sigma-pi* and pi-pi* states) and their nonplanar counterparts 2 (1)A(') and 3 (1)A('). Full geometry optimizations have been performed using analytic gradient techniques developed for the MR-CISD and MR-AQCC methods as implemented into the COLUMBUS program system. Basis set extrapolation techniques have been used for the determination of high-accuracy geometries and adiabatic excitation energies. Harmonic vibrational frequencies have been computed also. Agreement between calculated and available experimental data is very good. Especially for the sigma-pi* and pi-pi* states experimental information is extremely scarce and our results provide reliable predictions. The major new result of our work is the finding that for the 2 (1)A(1)(pi-pi*) state the structure optimized under planarity constraints is only a saddle point and not a minimum. This fact is the result of a conical intersection between the sigma-pi* and pi-pi* states (1 B-1(1) and 2 (1)A(1)). The final result is that neither on the 2 (1)A(') nor on the 3 (1)A(') surface a stationary minimum can be assigned to the pi-pi* state. (C) 2001 American Institute of Physics.