A PROGRAM SYSTEM FOR AB-INITIO MO CALCULATIONS ON VECTOR AND PARALLEL PROCESSING MACHINES .2. SCF CLOSED-SHELL AND OPEN-SHELL ITERATIONS

This series of three papers presents a program system for ab initio molecular orbital calculations on vector and parallel computers. Part II is devoted to SCF iterations on closed-shell and open-shell configurations starting from a file of two-electron integrals on the basis of contracted Gaussians (CGTOs). In a preliminary step, the two-electron integrals (pq{norm of matrix}rs) are reordered according to increasing values of index pq = p(p-1)/2+q. Then, in the first SCF iteration step, a file of semi-ordered P supermatrix elements (or P and Q supermatrix elements in the open-shell case) is generated from the file of semi-ordered integrals. This file is processed at each iteration step in an efficient vector loop to generate the electron repulsion matrix. Convergence is automatically controlled through level-shifting techniques. The most time-consuming parts are the integral sorting and the generation of the P supermatrix, which are carried out only once. Subsequent SCF iteration steps respectively require 0.9 s and 1.7 s for the process of 107 supermatrix elements by the closed-shell and the open-shell programs on a CRAY-2 processor. © 1990.