Geometry optimization of excited valence states of formaldehyde using analytical multireference configuration interaction singles and doubles and multireference averaged quadratic coupled-cluster gradients, and MR-AQCC gradients and the conical intersection formed by the 1{sup 1}B{sub 1}({sigma}-{pi}*) and 2{sup 1}A{sub 1}({pi}-{pi}*) 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 {sup 1}A{sub 1} (planar ground state), 1 {sup 1}A{sub 2} and 1 {sup 1}A{prime} (planar and nonplanar n-{pi}* state), 1 {sup 1}B{sub 1} and 2 {sup 1}A{sub 1} (planar {sigma}-{pi}* and {pi}-{pi}* states) and their nonplanar counterparts 2 {sup 1}A{prime} and 3 {sup 1}A{prime}. 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 {sup 1}A{sub 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 {sup 1}B{sub 1} and 2 {sup 1}A{sub 1}). The final result is that neither on the 2 {sup 1}A{prime} nor on the 3 {sup 1}A{prime} surface a stationary minimum can be assigned to the {pi}-{pi}* state.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC); FOR
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 943139
- Report Number(s):
- ANL/CHM/JA-37797; JCPSA6; TRN: US201002%%556
- Journal Information:
- J. Chem. Phys., Vol. 114, Issue 2 ; Jan. 8, 2001; ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- ENGLISH
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