Parallel Calculation of Coupled Cluster Singles and Doubles Wave Functions Using Array Files.

A new parallel implementation of the Coupled Cluster Singles and Doubles (CCSD) and related wave functions (e.g Quadratic Configuration Interaction, QCI, and Coupled Electron Pair, CEPA) is described, based on the Array Files middleware. The program can handle large basis sets, even without utilizing symmetry, on modest distributed memory workstation clusters. High computational efficiency is achieved by formulating all major operations in terms of matrix multiplications. Timings are provided for systems with 50-228 valence electrons and up to 1144 basis functions, with little or no symmetry. Our largest calculation (QCISD/aug-cc-pVQZ for the parallel displaced benzene dimer) uses 1512 basis functions. Calculations on the benzene dimer show that the usual procedure of estimating the effect of basis set enlargement from second-order Møller-Plesset (MP2) calculations is less reliable than previously assumed. Replacing the weak pair amplitudes in CCSD/QCISD calculations by MP2 amplitudes affects the calculated energy only slightly.