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Pseudopotential

Pseudopotential
Definition:Approximation of the Schrödinger equation exclusively for valence electrons.
Explanation:The pseudopotential is used to simplify the mathematical description of complex systems and is constructed to replace the full atomic all-electron potential such that core states are eliminated and the valence electrons of outer shells are described by pseudo-wave functions with significantly fewer nodes. This allows the pseudo-wave functions to be described with far fewer Fourier modes, thus making plane-wave basis sets practical to use. In this approach, only the chemically active valence electrons are usually explicitly dealt with, while the core electrons are “frozen”, being considered together with the nuclei as rigid non-polarizable ion cores. Ab initio pseudopotentials are derived from an atomic reference state, requiring that the pseudo- and all-electron valence eigen states have the same energies and amplitude outside a chosen core cutoff radius [1].
References:[1] http://en.wikipedia.org/wiki/Pseudopotential
See also, e.g. P. Schwerdtfeger, ChemPhysChem 12, 3143 (2011)