Molecular parameters of tetraatomic carbonyls X 2 CO and XYCO (X, Y = H, F, Cl) in the ground and lowest excited electronic states, part 1: A test of ab initio methods

Geometrical parameters of tetraatomic carbonyl molecules X 2 CO and XYCO (X, Y = H, F, Cl) in the ground ( S 0 ) and lowest excited singlet ( S 1 ) and triplet ( T 1 ) electronic states as well as values of barriers to inversion in S 1 and T 1 states and S 1 ← S 0 and T 1 ← S 0 adiabatic transition energies were systematically investigated by means of various quantum‐chemical techniques. The following methods were tested: HF, MP2, CIS, CISD, CCSD, EOM‐CCSD, CCSD(T), CR‐EOM‐CCSD(T), CASSCF, MR‐MP2, CASPT2, CASPT3, NEVPT2, MR‐CISD, and MR‐AQCC within cc‐pVTZ and cc‐pVQZ basis sets. The accuracy of quantum‐chemical methods was estimated in comparison with experimental data and rather accurate structures of excited electronic states were obtained. MP2 and CASPT2 methods appeared to be the most efficient and CCSD(T), CR‐EOM‐CCSD(T), and MR‐AQCC the most accurate. It was found that at equilibrium all the molecules under study are nonplanar in S 1 and T 1 electronic states with CO out‐of‐plane angle ranging from 34° (H 2 CO, S 1 ) to 52° (F 2 CO, T 1 ), and height of barrier to inversion varying from 300 (H 2 CO, S 1 ) to 11,000 (F 2 CO, T 1 ) cm −1 . © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009