Construction of open shell perturbation theory

Various forms of multi-reference perturbation theory, including our recently suggested ‘invariant’ form are compared for some diatomic supermolecules. While the ‘invariant’ zeroth-order Hamiltonian is not the direct sum of the zeroth-order fragment Hamiltonians, the size consistency errors in the final energies from using it often are no worse than for the other methods for the examples considered here.