Second-order Møller-Plesset calculations on the water molecule using Gaussian-type orbital and gaussian-type geminal theory

The Gaussian-type orbital and Gaussian-type geminal (GGn) model is applied to the water molecule, at the level of second-order Møller–Plesset (MP2) theory. In GGn theory, correlation factors are attached to all doubly-occupied orbital pairs (GG0), to all doubly-occupied and singly-excited pairs (GG1), or to all orbital pairs (GG2). Optimizing the GG2 model using a weak-orthogonality functional, we obtain the current best estimate of the all-electron MP2 correlation energy of water,!361.95 mEh. In agreement with previous observations, the GG1 model performs almost as well as the GG2 model (!361.26 mEh), whereas the GG0 model is poorer (!351.36 mEh). For the barrier to linearity of water, we obtain an MP2 correlation contribution of!463 " 5 cm!1. I