A PARALLEL VECTORIZED IMPLEMENTATION OF TRIPLE EXCITATIONS IN CCSD(T) - APPLICATION TO THE BINDING-ENERGIES OF THE ALH3, ALH2F, ALHF2 AND ALF3 DIMERS

An efficient method for evaluating various non-iterative estimates of connected triple excitations in coupled-cluster theory is outlined and related to a similar expression occurring in Moller-Plesset perturbation theory. The method is highly vectorized and capable of utilizing multiple processors on a shared-memory machine, leading to computational rates in excess of one billion floating-point operations per second on four processors of a CRAY Y-MP. Using the new procedure, the binding energies of the D2h diborane-type dimers of AlH3, AlH2F, AlHF2 and AlF3 have been determined to be 32, 40, 20 and 47 kcal/mol, respectively. For Al2F6, the correlation procedure includes 232 molecular orbitals and over 1.5 x 10(6) single and double coupled-cluster amplitudes, effectively accounting for over 2 x 10(9) connected triple excitations.