David P. Tew^a, Wim Klopper^*,a, Christian Neiss_{b, Christof Hättig^c
aLehrstuhl für Theoretische Chemie, Institut für Physikalische Chemie, Universität Karlsruhe (TH), D-76128 Karlsruhe, Germany
^bInstitut für Nanotechnologie, Forschungszentrum Karlsruhe, D-76344 Karlsruhe, Germany
^cLehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, D-44780 Bochum, Germany}

Phys. Chem. Chem. Phys., 9, 1921-1930 (2007).
Received 27th November 2006, Accepted 22nd December 2006
First published as an Advance Article on the web 21st February 2007

The explicitly-correlated coupled-cluster method CCSD(T)(R12) is extended to include F12 geminal basis functions that decay exponentially with the interelectronic distance and reproduce the form of the average Coulomb hole more accurately than linear-r₁₂. Equations derived using the Ansatz 2 strong orthogonality projector are presented. The convergence of the correlation energy with orbital basis set for the new CCSD(T)(F12) method is studied and found to be rapid, 98% of the basis set limit correlation energy is typically recovered using triple-zeta orbital basis sets. The performance for reaction enthalpies is assessed via a test set of 15 reactions involving 23 molecules. The title statement is found to hold equally true for total and relative correlation energies.

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