Using the MP2-R12 method

Although this manual is not a how-to on running quantum chemistry applications, the MP2-R12 method is a rather non-standard tool, hence a few comments on its use are appropriate.

1. The version of the MP2-R12 method implemented in PSI3 is a so-called single-basis MP2-R12 method in standard approximation A. This means that a basis set rather complete in Hartree-Fock (or one-particle) sense is absolutely mandatory for meaningful computations with the MP2-R12 method. The user is strongly urged to read literature on linear R12 methods before using PSI3 to compute MP2-R12 energies.
2. More robust, two-basis versions of the MP2-R12 method, also known as the auxiliary basis MP2-R12 method, have been implemented in a publicly available Massively Parallel Quantum Chemistry (MPQC) package (see http://aros.ca.sandia.gov/~cljanss/mpqc/). The two-basis version of the MP2-R12 method is a theoretically more sound approach, and thus should be preferred to the single-basis method. In some situations, however, it may make sense to use the single-basis method.