multiconfigurational

Multiconfigurational refers to electronic wavefunctions that are expressed as a linear combination of multiple configurations (Slater determinants) rather than a single determinant. This approach is used when a system exhibits near-degeneracy or strong static correlation among occupied and virtual orbitals, making single-reference methods inadequate.

In practice, the leading method is complete active space self-consistent field (CASSCF), part of the broader multiconfigurational self-consistent field (MCSCF) family. An active space of electrons and orbitals is selected to be treated exactly within all configurations in that space, while other electrons are kept frozen or treated approximately. Orbitals are optimized together with the configuration interaction coefficients, yielding a balanced description of ground and low-lying excited states. Multiconfigurational methods explicitly account for static correlation, which arises when several electronic configurations contribute significantly to the wavefunction.

Dynamic correlation outside the active space is typically added afterward with perturbation theory (CASPT2, NEVPT2) or multireference configuration interaction (MR-CI). These steps refine energies and properties by capturing short-range electron correlation neglected within the active space.

Applications commonly include bond dissociation and diradical systems, transition-metal complexes, and photochemical processes where conventional single-reference methods fail to describe near-degeneracy effects or excited-state landscapes. Limitations include the challenge of selecting an appropriate active space, rapid scaling of the configurational space with active space size, and method-specific complexity. Recent advances, such as density matrix renormalization group (DMRG) methods, enable larger active spaces and more flexible multiconfigurational analyses, broadening the scope of systems that can be treated accurately.