An efficient, fragment-based electronic structure method for molecular systems: Self-consistent polarization with perturbative two-body exchange and dispersion

We report a fragment-based electronic structure method, intended for the study of clusters and molecular liquids, that incorporates electronic polarization (induction) in a self-consistent fashion but treats intermolecular exchange and dispersion interactions perturbatively, as post-self-consistent field corrections, using a form of pairwise symmetry-adapted perturbation theory. The computational cost of the method scales quadratically as a function of the number of fragments (monomers), but could be made to scale linearly by exploiting distance-dependent thresholds. Extensive benchmark calculations are reported using the S22 database of high-level ab initio binding energies for dimers, and we find that average errors can be reduced to <1 kcal/mol with a suitable choice of basis set. Comparison to ab initio benchmarks for water clusters as large as (H2O)(20) demonstrates that the method recovers greater than or similar to 90% of the binding energy in these systems, at a tiny fraction of the computational cost. As such, this approach represents a promising path toward accurate, systematically improvable, and parameter-free simulation of molecular liquids. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3560026]