Parallel direct implementations of second‐order perturbation theories

Two algorithms are presented for parallel direct computation of energies with second‐order perturbation theory. Closed‐shell MP2 theory as well as the open‐shell perturbation theories OPT2(2) and ZAPT2 have been implemented. The algorithms are designed for distributed memory parallel computers. The first algorithm exhibits an excellent load balance and scales well when relatively few processors are used, but a large communication overhead reduces the efficiency for larger numbers of processors. The other algorithm employs very little interprocessor communication and scales well for large systems. In both implementations the memory requirement has been reduced by allowing the two‐electron integral transformation to be performed in multiple passes and by distributing the (partially) transformed integrals between processors. Results are presented for systems with up to 327 basis functions.