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

A general strategy for the calculation of energy differences is propos ed. It proceeds through the definition of a minimal model space and th e low-order perturbative development of the corresponding Hamiltonian is used to establish a set of determinants contributing to the searche d energy difference. The so-selected CI is treated variationally. This general strategy is applied here to the calculation of observables ba sically involving two electrons in two orbitals. The first one concern s 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 mo lecules (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 sati sfactory.