Improved algorithm for triple-excitation contributions within the coupled cluster approach

A new algorithm to handle triple-excitation contributions within the coupled cluster and/or many body perturbation theory is presented. It achieves a theoretical acceleration factor of 3 on a sub-set of the calculations, resulting for molecules in C-1 symmetry in a maximal speedup of 1.6 and 2.0 for open and closed shell cases, respectively. The algorithm is based on common footing for ppph - pphh and phhh - pphh interactions and on a tricky way of handling pre-summed sub-matrices, which enables one to decrease the number of similar to N-7 operations by converting them to similar to N-6. After a complete reorganization of the memory usage and handling of files, typical speedup factors of 1.6 - 1.8 are achieved for real calculations. Parallelization is straightforward with minimal communication requirements and scalability is excellent on distributed clusters.