A transformed framework for dynamic correlation in multireference problems

We describe how multireference dynamic correlation theories can be naturally obtained as singlereference correlation theories in a canonically transformed frame. Such canonically transformed correlation theories are very simple and involve identical expressions to their single-reference counterparts. The corresponding excitations involve quasiparticles rather than the bare particles of the system. High-order density matrices (or their approximations) and the numerical metric instabilities common to multireference correlation theories do not appear. As an example, we formulate the Bogoliubov canonically transformed version of second-order Moller-Plesset perturbation theory and demonstrate its performance in H-2, H2O, N-2, and BeH2 bond dissociation. (C) 2015 AIP Publishing LLC.