David P. Tew^1,a), Benjamin Helmich^2,b), Christof Hättig^2,c)
2 School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom
¹ Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, D-44780 Bochum, Germany

J. Chem. Phys. 135, 074107 (2011).
(Received 9 June 2011; accepted 14 July 2011; published online 17 August 2011)

We explore using a pair natural orbital analysis of approximate first-order pair functions as a means to truncate the space of both virtual and complementary auxiliary orbitals in the context of explicitly correlated F12 methods using localised occupied orbitals. We demonstrate that this offers an attractive procedure and that only 10-40 virtual orbitals per significant pair are required to obtain second-order valence correlation energies to within 1-2% of the basis set limit. Moreover, for this level of virtual truncation, only 10-40 complementary auxiliary orbitals per pair are required for an accurate resolution of the identity in the computation of the three- and four-electron integrals that arise in explicitly correlated methods. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624370]

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