Parallel calculation of coupled cluster energies using array files

by Janowski, Tomasz; Ford, Alan Ray; Pulay, Peter

We have developed an efficient full-accuracy parallel CCSD/QCISD program for moderate-sized PC clusters. It uses the SCEP matrix formulation, and the efficient generator state spin adaptation. The virtual space is described by either AOs or MOs. Only integral with two virtual indexes are transformed and stored, resulting in modest disk usage. Because of the superior performance of matrix multiplication on modern CPUs, the algorithm is formulated mostly as matrix multiplications. Parallelization is based on Array Files (AF), a simple parallel, portable file system. AF transparently stores and retrieves the arrays needed in CCSD, such as amplitude and integral matrixes, and greatly simplifies the implementation, without lowering the performance. Our new program can perform calcns. of unprecedented size, such as a QCISD calcn. with 1512 basis functions and no symmetry on a 20-processor cluster. Current work centers on neglecting or approximating weak pairs, using symmetry, and adding perturbative triple substitution corrections.