Diagrammatic CASPT2 using an internally contracted basis

The diagrammatic CASPT2 method is derived for an internally contracted basis. The wave-operator is efficient to compute and does not require orthogonalizing the internally contracted basis set and diagonalizing the zeroth-order Hamiltonian H0. The wave-operator also has a simple form that is a generalization of Møller–Plesset perturbation theory.