17.1 Overview: sets and the basis library

Basis functions are used in Molpro not just for representing orbitals, but also for providing auxiliary sets for density fitting (see 11) and for simplifying integrals through approximate identity resolution in explicitly-correlated methods (see 29). In order to accommodate this, the program maintains internally a number of different sets. The first of these always has the name ORBITAL and is the primary basis set for representing orbitals, and others can be defined as necessary as described below, or else are constructed automatically by the program when required. In the latter case, the density-fitting and other modules attempt to guess a reasonable libary fitting basis that should be appropriate for the orbital basis set; it is advisable to check the choice when using anything other than a standard orbital basis set.

The basis sets may either be taken from the program library, or may be specified explicitly, or any combination. Optionally, the basis function type can be chosen using the CARTESIAN or SPHERICAL commands.