corevalence

Corevalence, or core–valence, describes the division of electrons in atoms and molecules into core (inner-shell) and valence (outer-shell) groups. Core electrons are tightly bound and largely nonbonding, while valence electrons participate in chemical bonding and determine reactivity. The core–valence separation is a practical framework in both qualitative descriptions and quantitative calculations, reflecting the energy gap between core and valence orbitals in most elements.

In quantum chemistry, core–valence separation (CVS) is an approximation used in the calculation of electronic excited states and core-level spectra. In CVS, excitations are restricted to involve core orbitals in a controlled way, or the core electrons are treated separately from valence electrons. This reduces the size of the active space and computational cost while enabling accurate modeling of core-excitation phenomena, such as X-ray absorption spectroscopy (XAS) and near-edge X-ray absorption fine structure (NEXAFS). Complementary approaches include freezing core orbitals or using effective core potentials (pseudopotentials) to replace core electrons with a simplified potential.

Core–valence correlation refers to electron correlation effects that span core and valence electrons. Accounting for core–valence correlation can improve accuracy for properties sensitive to core orbitals, such as core-ionization energies and core-excitation energies, but it increases computational demands and is not always necessary for valence-focused properties.

Applications span inorganic and organic chemistry, catalysis, and materials science, particularly where core-level spectroscopy provides structural or electronic information about a system, or where accurate core-level energies are needed for comparison with experiment.