On the size extensivity of second-order Møller–Plesset perturbation theory based on a perfectly-paired generalized valence bond reference wavefunction Original Research Article

This paper examines the size extensivity property of second-order Møller–Plesset perturbation theory based on a spin-zero, perfectly-paired generalized valence bond reference wavefunction (GVB-MP2). Size extensivity is a desirable property of any approximate method. The GVB-MP2 energy can be evaluated using a contracted formulation or by using the uncontracted formulation. This paper shows that the contracted formulation of GVB-MP2 is size extensive, while the uncontracted implementation of the perturbation theory is not size extensive. The paper also shows that the uncontracted formulation of GVB-MP2 yields results in agreement with expectations based on the cluster expansion of the energy.