1ST-ORDER PROPERTIES AND THE HELLMANN-FEYNMAN THEOREM IN THE CASE OF A LIMITED CI WAVE-FUNCTION

Two distinct approaches to the calculation of first‐order properties with a limited CI wave function are discussed. One is based on the Hellmann–Feynman theorem and the other on the direct evaluation of the total energy derivative at zero perturbation. Corrections to the Hellmann–Feynman expectation value are given for the CI wave function consisting of a single determinant reference state and all single and double replacements of this. These corrections are the extended Brillouin matrix elements and involve interactions between the zeroth‐order wave function and triply substituted configurations. The usefulness of these matrix elements for the generation of MC SCF orbitals and for the calculation of cluster corrections to the wave function is briefly discussed. The formulas for the Brillouin matrix elements expressed in terms of one‐ and two‐electron integrals have been automatically generated using the syntax of the algebraic program SCHOONSCHIP. Copyright © 1979 John Wiley & Sons, Inc.