Accurate and Efficient Quantum Chemistry Calculations for Noncovalent Interactions in Many-Body Systems: The XSAPT Family of Methods

We present an overview of “XSAPT”, a family
of quantum chemistry methods for noncovalent interactions. These methods
combine an efficient, iterative, monomer-based approach to computing
many-body polarization interactions with a two-body version of symmetry-adapted
perturbation theory (SAPT). The result is an efficient method for
computing accurate intermolecular interaction energies in large noncovalent
assemblies such as molecular and ionic clusters, molecular crystals,
clathrates, or protein–ligand complexes. As in traditional
SAPT, the XSAPT energy is decomposable into physically meaningful
components. Dispersion interactions are problematic in traditional
low-order SAPT, and two new approaches are introduced here in an attempt
to improve this situation: (1) third-generation empirical atom–atom
dispersion potentials, and (2) an empirically scaled version of second-order
SAPT dispersion. Comparison to high-level ab initio benchmarks for dimers, water clusters, halide–water clusters,
a methane clathrate hydrate, and a DNA intercalation complex illustrate
both the accuracy of XSAPT-based methods as well as their limitations.
The computational cost of XSAPT scales as O­(N3)–O­(N5) with respect to monomer size, N, depending
upon the particular version that is employed, but the accuracy is
typically superior to alternative ab initio methods
with similar scaling. Moreover, the monomer-based nature of XSAPT
calculations makes them trivially parallelizable, such that wall times
scale linearly with respect to the number of monomer units. XSAPT-based
methods thus open the door to both qualitative and quantitative studies
of noncovalent interactions in clusters, biomolecules, and condensed-phase
systems.