A METHOD TO REDUCE THE NUMBER OF 2 ELECTRON INTEGRAL TRANSFORMATIONS IN A 2ND-ORDER MULTICONFIGURATIONAL HARTREE-FOCK PROCEDURE

In standard multiconfigurational Hartree-Fock (MCSCF) theory, a two electron integral transformation is performed before application of the equation defining the MCSCF approach in each iterative cycle. It is shown how the two electron integral transformation may be replaced by an approximate orbital transformation introduced directly into the equation that defines the second order MCSCF approach. In this way, the number of two electron integral transformations required to obtain a set of MCSCF orbitals is reduced considerably. Numerical examples for the 3Σg -, 1Δg, and 1Σ g+ states of O2 indicate that an accuracy of 10-6 a.u. in the total energy may be obtained by carrying out 2-3 two electron integral transformations, which is about half the number of transformations required to obtain the same accuracy in the second order MCSCF approach. An accuracy of 10-10 a.u. is obtained after one further iteration is carried out with a second order MCSCF scheme. © 1980 American Institute of Physics.