Effective Algorithms of the RI Approximation for the CIS Method: an Example of Application of the High-Memory Strategy in the Ab Initio Calculations

Two variants of the CIS methods with the RI approximation have been implemented. Both methods employ the high-memory strategy: the first, RI-CIS(1), is based on the full storage of the decomposed electronic repulsion integrals (ERI) tensor and CIS Hamiltonian, while the second, RI-CIS(2), stores only the decomposed ERI tensor. Both variants of the RI-CIS were tested for parallelism, performance, and precision. The results are compared with the default CIS method and RIJCOSX approximation. The considered methods demonstrate higher performance compared to their analogs and higher precision compared to the RIJCOSX approximation. Even the worse scaling of the RI methods did not lead to the lower performance in the conducted test calculations. The reported algorithms show that the performance of the quantum chemistry calculations is limited not only by the CPU power but also by the availability of RAM. The large volume of available memory can significantly increase the speed of the calculation by employing more effective but memory-consuming algorithms.