Off-center Gaussian functions: Applications toward larger basis sets, post-second-order correlation treatment, and truncated virtual orbital space in investigations of noncovalent interactions

This work is a continuation of our pilot study on the use of off-center s-type Gaussian functions in noncovalent interaction calculations. A grid of s-type Gaussians surrounding the molecule is intended to substitute for the presence of diffuse basis functions, which are important for accurate description of noncovalent interactions. The advantage over the use of diffuse functions is reduction or elimination of linear dependency issues in the atomic orbital basis set, which often causes convergence problems. Placement of a grid of s-functions on the surface of the molecule allows more favorable scaling of the total number of basis functions with the molecular size to be achieved. In this article, we present several innovations on the concept, namely the parametrization and assessment of grids with triple- quality basis set; use in post-second-order correlation treatment (methods such as MOller-Plesset third-order or coupled-cluster); and combination with modified virtual orbital approaches, namely the frozen natural orbitals and optimized virtual orbital space methods.