N-electron valence state perturbation theory: Difference between revisions

Revision as of 18:59, 9 February 2006

Revision as of 21:27, 2 March 2006

Line 26:

To be continued...

== Properties ==

NEVPT is blessed with many important properties, making the approach very solid and reliable. These properties arise both from the theoretical approach used and on the Dyall's Hamiltonian particular structure:

* '''[[Size consistency]]''': NEVPT is [[size consistency|size consistent]] ([[strict separability|strict separable]]). Briefly, if A and B are two non-interacting systems, the energy of the supersystem A-B is equal to the sum of the energy of A plus the energy of B taken by themselves (<math>E(A-B) = E(A) + E(B)</math>). This property is of particular importance to obtain correctly behaving dissociation curves.

* '''Absence of [[Intruder state|intruder states]]''': in perturbation theory, divergencies can occur if the energy of some perturber happens to be nearly equal to the energy of the zero-order wavefunction. This situation, which is due to the presence of an energy difference at the denominator, can be avoided if the energies associated to the perturbers are guaranteed to be never nearly equal to the zero-order energy. Møller-Plesset perturbation theory satisfies this requisite, the energy difference being <math>\epsilon_{r}+\epsilon_{s}-\epsilon_{i}-\epsilon_{j}</math>, with <math>\epsilon</math> orbital energies of the virtual (<math>r,s</math> indexes) and core (<math>i,j</math> indexes). NEVPT also satisfies this requirement, with a resulting expression for the energy differences conceptually similar to the MP2.

* '''Invariance under active orbital rotation''': The NEVPT results are stable if an intraclass active-active orbital mixing occurs. This arises both from the structure of the Dyall Hamiltonian and the properties of a CASSCF wavefunction. This property has been also extended to the intraclass core-core and virtual-virtual mixing, thanks to the Non Canonical NEVPT approach, allowing to apply a NEVPT evaluation without performing an orbital canonization (which is required, as we saw previously)

* '''Spin purity is guaranteed''': The resulting wavefunctions are guaranteed to be spin pure, due to the spin-free formalism.

== See also ==