Fragment-based electronic structure methods are becoming popular as a means to extend quantum chemistry to large systems, and to take advantage of massively-parallel computing resources.  The accuracy of many of these methods is less than desirable, however; errors of several kcal/mol (with respect to supersystem benchmarks) are not uncommon.  This talk will describe several methods developed recently in our group that are aimed at achieving high accuracy (errors < 1 kcal/mol) with respect to the best-available supersystem benchmarks, typically MP2 or CCSD(T) extrapolated to the basis-set limit.  One such method is a many-body generalization of symmetry adapted perturbation theory that we call XSAPT.  In conjunction with Kohn-Sham monomer orbitals and empirical dispersion potentials, the 'XSAPT(KS)+D' method achieves this level of accuracy and also extends SAPT-based energy decomposition analysis to many-body systems.  A second method is based on a generalization of the many-body expansion to exploit overlapping fragments. We have used this approach to achieve high accuracy in challenging test cases, in polyatomic systems, and to extend fragment-based methods to the large basis sets that are required to eliminate basis set superposition errors.  Efficient counterpoise corrections for many-body systems have also been developed along these lines.

Fast and accurate electronic structure calculations for large systems

246th National Meeting (2013)

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