On the development and implementation of multi-CPU parallel versions of accurate, general purpose, methods of multireference perturbation theories

Abstract

Algorithms, and pilot implementations, of multi-CPU parallel versions of the second-order Generalized Van Vleck Perturbation Theory method for molecular electronic structure (GVVPT2) and of the perturbatively corrected multireference configuration interaction method including single and double electron replacements [nR-MRCISD(TQ)] are presented and analyzed. It is shown that a cornerstone of the organization of the original, serial methods, macroconfigurations, is also effective for organizing parallel versions. In fact, the use of macroconfigurations, and specifically pairs of macroconfigurations between interacting subspaces, suggests a natural framework that allows optimization of the parallel algorithms. As with the earlier serial versions, a graphical implementation of orbital configurations, which in turn allows efficient use of unitary group approach coupling coefficients, runs below organization at the macroconfiguration level. Consequently, resulting wave functions are rigorously spin adapted, and spatial symmetry has also been implemented at the level of Abelian point groups. Numerical results are presented and analyzed for illumination of future directions for improvement. The parallelization scheme is oriented to departmental and university class computer clusters (or supercomputers) and is expected to facilitate the effective use of GVVPT2 for many molecules of interest to computational chemistry, and shows that the highly accurate nR-MRCISD(TQ) can now be considered as available for investigating problematic molecules outside the applicability of more commonly available methods.