Vectorizing a sequence of conditional branches: The calculation of the class index of two-electron repulsion integrals on cray computers
The relative advantages of processing a supermatrix or a two-electron integral file for the vector-oriented computation of two-electron interaction matrices in ab initio quantum chemical calculations are discussed briefly. It is concluded that the supermatrix formalism is less appropriate for large molecules. Two vector algorithms are presented for the calculation of the class index characterizing each two-electron repulsion integral. Benchmark tests have been carried out on a CRAY-2 computer; the timings for the new vector algorithms and for the standard scalar computations are reported for the two presently available versions of the CRAY-2 FORTRAN compiler (CFT 77 and CFT-2).