Abstract
Excited states of C₂, H₂O, CO, and formamide are studied to illustrate the performance and accuracy of second-order diagrammatic complete active space perturbation theory (D-CASPT2) [J. Chem. Phys. 108, 1081 (1998)]. Comparisons are made with other ab initio methods and also full configuration interaction (FCI) calculations or experiment. Excitation energies computed by the D-CASPT2 method are quite accurate showing an average deviation of 0.1eV from the FCI values for C₂ and H₂O. The CO and formamide excitation energies yield average deviations of 0.1 and 0.2 eV from experiment, respectively. The computational cost of this method is reduced to a great extent compared to the MRMP method.