The previously developed DFT-SAPT approach, which combines symmetry-adapted intermolecular perturbation theory (SAPT) with a density-functional theory (DFT) representation of the monomers, has been implemented by using density fitting of two-electron objects. This approach, termed DF-DFT-SAPT, scales with the fifth power of the molecular size and with the third power upon increase of the basis set size for a given dimer, thus drastically reducing the cost of the conventional DFT-SAPT method. The accuracy of the density fitting approximation has been tested for the ethyne dimer. It has been found that the errors in the interaction energies due to density fitting are below 10(-3) kcal/mol with suitable auxiliary basis sets and thus one or two orders of magnitude smaller than the errors due to the use of a limited atomic orbital basis set. An investigation of three prominent structures of the benzene dimer, namely, the T shaped, parallel displaced, and sandwich geometries, employing basis sets of up to augmented quadruple-zeta quality shows that DF-DFT-SAPT outperforms second-order Moller-Plesset theory (MP2) and gives total interaction energies which are close to the best estimates inferred from combining the results of MP2 and coupled-cluster theory with single, double, and perturbative triple excitations.

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An adiabatic-connection fluctuation-dissipation theorem approach based on a range separation of electron-electron interactions is proposed. It involves a rigorous combination of short-range density functional and long-range random phase approximations. This method corrects several shortcomings of the standard random phase approximation and it is particularly well suited for describing weaklybound van der Waals systems, as demonstrated on the challenging cases of the dimers Be2 and Ne2.Density functional theory (DFT) is a powerful approach for electronic-structure calculations of molecular and condensed-matter systems [1]. However, one difficulty in its Kohn-Sham (KS) formulation using local density or generalized-gradient approximations (LDA and GGA) is the description of non-local correlation effects, such as those involved in weak van der Waals complexes, bound by London dispersion forces [2]. The adiabaticconnection fluctuation-dissipation theorem (ACFDT) approach is one of the most promising ways of constructing highly non-local correlation functionals. This approach, introduced in wave function theory [3] and in DFT [4,5], consists in extracting non-local ground-state correlations from the linear charge density response function.Recently, the ACFDT approach has received renewed interest for implementing the random phase approximation (RPA) or other related approximations for atoms, molecules and solids [6][7][8][9][10][11]. The RPA correlation energy is consistent with the use of the exact, self-interactionfree exchange energy. In spite of a number of encouraging results, such as the correct description of dispersion forces at large separation [12], the proper reproduction of cohesive energies and lattice constants of solids [10,13,14] and an improved description of bond dissociation [6,7,15], several aspects of the RPA are still unsatisfactory.First, the RPA is a poor approximation to short-range correlations, leading to correlation energies that are far too negative [16]. Second, in a Gaussian localized basis, RPA calculations have a slow convergence with respect to the basis size [6]. Third, the presence of an unphysical maximum (bump) at medium distances in dissociation curves of simple diatomic molecules [6,15] indicates an inherent problem which has not yet a fully clarified origin. Fourth, although in principle the orbitals should be calculated self-consistently [17], most RPA implementations consist of a post-KS single-iteration calculation, making the choice of the input orbitals sometimes critical. Last * Electronic address: julien.toulouse@upmc.fr † Electronic address: janos.angyan@crm2.uhp-nancy.fr but not least, although the main advantage of the RPA is supposed to be the description of dispersion forces, rare gas dimer potential curves calculated from LDA or GGA orbitals are often qualitatively wrong, as shown later.The poor short-range behavior can be corrected by adding a GGA functional constructed from the difference of the exact and RPA correlation energies of the uniform electron gas [16]...

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