Molecular parameters of tetraatomic carbonyls X₂CO and XYGO (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 X₂CO and XYCO (X, Y = H, F, Cl) in the ground (S₀) and lowest excited singlet (S₁) and triplet (T₁) electronic states as well as values of barriers to inversion in S₁ and T₁ states and S₁ ← S_c and T₁ → S₀ adiabatic transition energies were systematically investigated by means of various quantumchemical techniques. The following methods were tested: HF, MP2, CIS, CISD, CCSD, EOMCCSD, CCSD(T), CR-EOM-CCSD(T), CASSCF, MR-MP2, CASPT2, CASPT3, NEVPT2, MRCISD, and MR-AQCC within cc-pVTZ and cc-pVQZ basis sets. The accuracy of quantumchemical 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 S₁ and T₁ electronic states with CO out-of-plane angle ranging from 34° (H₂CO, S₁) to 52° (F₂CO, T₁), and height of barrier to inversion varying from 300 (H₂CO, S ₁) to 11,000 (F₂CO. T₁) cm^-1.