SPECIFIC CI CALCULATION OF ENERGY DIFFERENCES - TRANSITION ENERGIES AND BOND-ENERGIES

A general strategy for the calculation of energy differences is proposed. It proceeds through the definition of a minimal model space and the low-order perturbative development of the corresponding Hamiltonian is used to establish a set of determinants contributing to the searched energy difference. The so-selected CI is treated variationally. This general strategy is applied here to the calculation of observables basically involving two electrons in two orbitals. The first one concerns the transition energies from the ground state to the lowest singlet and triplet states of atoms (Ar S-1-->P-3,P-1, Ca S-1-->P-3,P-1) or molecules (CH2 1A1-->B-3(1), 2 1A1). The second problem concerns the C-H bond energy in ethylene. In all cases the agreement of the results of that simple procedure with either experiment or full-CI is quite satisfactory.