Diagrammatic complete active space perturbation theory: Calculations on benzene, N2, and LiF

Second-order diagrammatic complete active space perturbation theory (D-CASPT2) is designed to have the best qualities from multireference Mo/ller–Plesset (MRMP) theory, complete active space perturbation theory CASPT2, and the effective valence shell Hamiltonian Hv method, including possessing a linked diagram theorem (LDT) and an efficient second-order formalism. The accuracy of this method is illustrated through calculations of the electronic excitation energies of N2 and benzene, the spectroscopic constants of the N2 ground state, and the ionic-neutral avoided crossing in the potential curves of LiF.