Perturbation theory
Perturbation theory gives correlated energies, but no new wavefunctions. PT is not variational. This means the calculated energy is not an upper bound for the exact energy. It is possible to partition Møller–Plesset perturbation theory energies via Interacting Quantum Atoms (IQA) energy partitioning (although most commonly the correlation energy is not partitioned). This is an extension to the theory of Atoms in Molecules. IQA energy partitioning enables one to look in detail at the correlation energy contributions from individual atoms and atomic interactions.