COUPLED-CLUSTER THEORY EMPLOYING APPROXIMATE INTEGRALS - AN APPROACH TO AVOID THE INPUT OUTPUT AND STORAGE BOTTLENECKS/

By representing orbital products in an expansion basis, certain classe s of two-electron integrals are approximated for use in CCSD(T) calcul ations (singles and doubles coupled-cluster plus a perturbational esti mate of the effects of connected triple excitations). This leads to a very large reduction in disk storage and input/output requirements, wi th usually only a modest increase in computational effort. The new pro cedure will allow very large CCSD(T) calculations to be undertaken, li mited only by available processor time, Using the molecular basis as t he expansion basis, explicit numerical comparisons of equilibrium geom etries, harmonic frequencies, and energy differences indicate that the error due to the use of approximate integrals is less than the error associated with truncation of the molecular basis set.