Parallel Calculation of CCSDT and Mk-MRCCSDT Energies

A scheme for the parallel calculation of energies at the coupled-cluster singles, doubles, and triples (CCSDT) level of theory, several approximate iterative CCSDT schemes (CCSDT-1a, CCSDT-1b, CCSDT-2, CCSDT-3, and CC3), and for the state-specific multireference coupled-cluster ansatz suggested by Mukherjee with a full treatment of triple excitations (Mk-MRCCSDT) is presented. The proposed scheme is based on the adaptation of a highly efficient serial coupled-cluster code leading to a communication-minimized implementation by parallelizing the time-determining steps. The parallel algorithm is tailored for affordable cluster architectures connected by standard communication networks such as Gigabit Ethernet. In this way, CCSDT and Mk-MRCCSDT computations become feasible even for larger molecular systems and basis sets. An analysis of the time-determining steps for CCSDT and Mk-MRCCSDT, namely the computation of the triple-excitation amplitudes and their individual contributions, is carried out. Benchmark calculations are presented for the N2O, ozone, and benzene molecules, proving that the parallelization of these steps is sufficient to obtain an efficient parallel scheme. A first application to the case of 2,6-pyridyne using a triple-ζ quality basis (222 basis functions) is presented demonstrating the efficiency of the current implementation.