Simplified reference wave functions for multireference perturbation theory.

A number of simplifications in defining the reference wave functions used in multireference second-order Møller-Plesset perturbation theory (MRMP2) calculations are studied. The usual multiconfigurational orbital optimization is avoided by using Hartree-Fock or Kohn-Sham orbitals; the complete configuration expansion in the active-space orbitals is replaced by a severely truncated expansion, and the spin-component-scaling idea is applied to the multireference perturbation expansion. We assess these approximations to the full procedure by calculating the barrier heights for 15 processes taken from the Zhao-Gonzalez-Garcia-Truhlar database. Our results suggest that reliable and relatively cheap reference wave functions for MRMP2 calculations can be obtained from the simplifications introduced here. We hope that this will enable the application of the MRMP2 method to a larger range of chemical systems.