PASSING THE SEVERAL BILLIONS LIMIT IN FCI CALCULATIONS ON A MINICOMPUTER

An efficient realization of a determinant-based algorithm for the larg e full configuration interaction (FCI) problem is presented; it effect ively exploits the sparsity of the CI vector. Within this technique we reproduced the one billion calculation for the S-1 ground state of th e Mg atom on a Spare IPC minicomputer to an uncertainty of 10(-4) hart ree within a threshold of 10(-5) for the CI vector. Calculations are a lso performed for the P-1 excited state of Mg. As an example, which is at the limit of the hardware available on our computer, the ground st ate of the H2O molecule in the [5s3p1d/3s1p] ANO basis set is calculat ed. The FCI expansion of the ground state vector yields over 7.2 billi on C-2v determinants. A ground state energy of - 76.305 hartree with a n estimated accuracy of 2 X 10(-3) hartree in this basis set is obtain ed within the total time of 170 h. One iteration of the CI vector in t his basis set with a threshold of 10(-5) is performed in 100 Mbyte dis k space and took 4200 min of the total computation time.