Ab initio wave-function(WF)-based methods can predict intermolecular force fields very accurately for monomers containing a few atoms, but applications to systems with tens of atoma are too time consuming. The density-functional theory (DFT) methods would be fast enough, but are currently not able to predict the very important dispersion component of the force field. We have developed a method based on a DFT description of monomers but computing intermolecular forces using expressions beyond DFT, originating from symmetry-adapted perturbation theory (SAPT). To obtain accurate predictions, it was necessary to fix the wrong long-range behavior of DFT monomer densities by applying an asymptotic correction to the exchange-correlation potential. The dispersion energies are computed from coupled Kohn-Sham frequency-dependent susceptibility functions. SAPT(DFT) calculations require only a small fraction of computer resources used by the regular SAPT and converge much faster in the size of the basis sets. Calculations for model compounds have shown that the new method reproduces all components of the intermolecular force, including dispersion, extremely well. Moreover, although initially SAPT(DFT) was expected to be a method providing medium quality results for very large molecules, it turned out that in some cases the accuracy of SAPT(DFT) surpasses that which can be reached with the currently programmed WF-based SAPT and reasonable size basis sets. The method is able to handle interactions of molecules containing 20 and more atoms. It has already been applied to several fairly large system like the benzen dimer or the RDX dimer (42 atoms). Examples of applications will be presented.
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