Q. How to determine the memory size with the option “%mem” for SAC-CI calculation?

 

A. To achieve good performance in the SAC/SAC-CI, it is recommended to use appropriate memory size for the system. In SAC/SAC-CI, MOERIs (MO Electron Repulsion Integrals) are stored “in-core” for fast random accesses in calculating Hamiltonian matrix elements. Since the MOERI usually dominates the core memory in the large systems, the following remarks may be informative.

(1) ERIs that are zero due to the symmetry are automatically omitted.

(2) When the size of the ERIs is larger than the core memory of a computer, a bucket-type algorithm is adopted, which is effective only for the SD-R part. For details, see explanation in #SDPRES# in the SAC-CI guide.

(3) The size of ERIs stored in core is approximately 8*(N*(N-1)/2)/Nrep words, where N=Nact*(Nact-1)/2, Nact is the active space of SAC/SAC-CI and Nrep is the number of irreducible symmetry representation. Memory size should be increased using %MEM or -M-, in large calculations, especially when the message “Recommend to increase core memory” appears.