Parallel direct four-index transformations

Two parallel direct integral transformation algorithms are presented. Specific attention is directed to producing transformed integrals containing at least two "active orbital" indices. The number of active orbitals is typically much less than the total number of molecular orbitals reflecting the requirements of a wide range of correlated electronic structure methods. Sample direct second-order Møller-Plesset theory calculations are reported. For situations where multipassing of the integrals is required, superlinear speedup is obtained by exploiting the increase in global memory. As a consequence, for morphine in a 6-31G basis, a speedup of over 25 is observed in scaling from 32 to 512 processors.