A one billion determinant full CI benchmark on the Cray T3D

The implementation of an out-of-core version of a full CI algorithm on the Cray T3D is described. The introduction of heavy I/O activity, necessary to handle larger problems, required particular attention in order to maintain good performance. As an application, the FCI energy of the Be2 molecule with a [9s2p1d] basis set (all electrons), whose FCI space has a dimension of more than one billion (109) of symmetry-adapted determinants in D2h symmetry was computed. A single iteration on the Cray T3D at CINECA (64 processors) required about four hours, 30 minutes of which were spent in I/O operations. 27 iterations were performed and a precision of at least one μhartree in the energy was achieved. Due to the scalability of the code, substantially larger calculations could be performed provided that more processors and a larger amount of disk space were available.