A Perspective on the CASPT2 Method

The CASPT2 method, developed by Roos and coworkers, is reviewed. The rationale for a method that can deal with arbitrary strongly correlated systems at affordable computational cost is presented, and the main milestones in the 20-year history of CASPT2 are listed. The basic features of the method are summarized. The strengths and weaknesses of CASPT2 are assessed. Its main strength is its ability to deal with arbitrary electronic states at a reasonable computational cost, and giving exact spin states. Its main limitation is that, it is not a black box method, and requires considerable experience to use. Another limitation is that, for accurate results, the active space must be generally larger than the minimum required to describe only the strong (static) correlation. Including dynamical correlation at an earlier stage may resolve some of these issues. (C) 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111: 3273-3279, 2011