Dual-Basis Pairings for Non-Covalent Interactions
While we typically view chemistry as the making and breaking of covalent bonds, much of chemistry (and biology and materials...) is dictated by non-covalent interactions. The properties of liquid water, for example, are strongly dictated by the interaction of the dipole moments of water molecules. These same interactions are responsible for the solvation of biomolecules.

Describing such interactions from a quantum chemical perspective, however, is extremely challenging. With several subtle, competing components, non-covalent interactions typically necessitate accurate electron correlation and, therefore, large basis sets. Dispersion interactions, for example, are inherently an electron correlation effect that cannot be described by mean-field calculations.

The fact that calculations for these systems typically reside in the large basis set regime makes them particularly attractive targets for dual-basis methods. Since large-basis MP2 is a standard routine for non-covalent complexes, the dual-basis analogue provides a cost-effective tool for systems that are otherwise intractable.

The long-range nature of these interactions makes diffuse ("augmented") basis sets appropriate and often more quickly convergent than higher angular momentum functions. Accordingly, we constructed and exhaustively tested pairings for the Dunning-style aug-cc-pV{D,T,Q}Z series of basis sets. The balanced structure of these basis sets, along with their associated dual-basis truncations, is shown below:

Results were shown to be extremely accurate—on the order of a few hundredths of a kcal/mol in energy and a couple thousandths of an Angstrom in structure—and provided cost savings of up to 95%.


Reference:
"Non-Covalent Interactions with Dual-Basis Methods: Pairings for Augmented Basis Sets" R. P. Steele, R. A. DiStasio, Jr., and M. Head-Gordon. J. Chem. Theor. Comput. 5 1560 (2009).