NEVPT

NEVPT, or N-electron valence state perturbation theory, is a post-CASSCF multireference perturbation theory used to recover dynamic electron correlation in systems with strong static correlation. It provides a size-consistent energy correction to a multireference wavefunction and is designed to be robust for near-degenerate electronic states.

The method employs Dyall's zero-order Hamiltonian to partition the electronic problem into core, active (valence), and virtual spaces. By constructing perturbative corrections on top of a CASSCF reference, NEVPT yields total energies, excitation energies, and potential energy surfaces that account for dynamic correlation while maintaining a stable description of multireference character. A key feature is its intruder-state resistance, arising from the chosen zeroth-order Hamiltonian and the structure of the perturber spaces.

Two main contracted forms exist: strongly contracted NEVPT (SC-NEVPT) and partially contracted NEVPT (PC-NEVPT). SC-NEVPT compresses amplitudes within each perturber space to compact expressions, offering efficiency, while PC-NEVPT retains more configurations for potentially higher accuracy at increased cost.

NEVPT is commonly used as an alternative to CASPT2 for multireference systems, including transition-metal complexes and excited-state problems, where reliable treatment of both static and dynamic correlation is essential. It can be applied to ground and excited states and is compatible with derivative and gradient calculations for geometry optimization and mapping potential energy surfaces.

See also CASPT2, multireference perturbation theory, and Dyall Hamiltonian.