Should the standard basis sets be augmented with diffuse functions on hydrogens to provide a reasonable description of the lowest Rydberg state of hydrogen-containing molecules?

Abstract

Augmentation of the standard basis sets with the diffuse functions, which were optimized for non-Rydberg anions on the heavy atoms only, may not be adequate for calculating Rydberg states of molecules. Addition of diffuse functions on the hydrogens to such a basis set used for calculations of Rydberg states of hydrogen-containing species can make up for this deficiency and lead to a reasonable description. This gives the possibility of using the standard basis sets (e.g., the 6-31G(d,p) and 6-311G(d,p) basis sets) augmented by diffuse functions, both on the heavy atoms and on the hydrogen atoms, for these calculations and, therefore, provides an alternative to optimizing diffuse functions specifically to describe Rydberg orbitals. Inclusion of diffuse s functions on hydrogens is less important in calculations of non-Rydberg hydrogen-containing anions.