Approximate procedures in multireference perturbation theory: Successes and limitations

We study a number of simplifications to the MRMP2 variant of multireference perturbation theory, with a view to enabling this type of methodology to be applied to a wider range of systems. We consider in turn: The elimination of the orbital optimization stage in preparing the reference wavefunction; the truncation of the configuration expansion in the active space; the combination of the two previous approximations and finally the spin-component scaling of the dynamic correlation energy that has proved very advantageous in single reference MP2 applications. These approximations have important formal and practical consequences, which we illustrate. We test these schemes on diatomic potential energy curves and polyatomic reaction barriers. While the approximations introduced are not applicable in all circumstances we fmd that many commonly encountered situations are amenable to treatment by these simplified procedures. Accordingly it is hoped that the realm of application of the MRMP2 method will be increased. © 2007 American Institute of Physics.