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 0.1 eV 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. © 2000 American Institute of Physics.