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Spin contamination : ウィキペディア英語版
Spin contamination
In computational chemistry, spin contamination is the artificial mixing of different electronic spin-states. This can occur when an approximate orbital-based wave function is represented in an unrestricted form – that is, when the spatial parts of α and β spin-orbitals are permitted to differ. Approximate wave functions with a high degree of spin contamination are undesirable. In particular, they are not eigenfunctions of the total spin-squared operator, ''Ŝ''2, but can formally be expanded in terms of pure spin states of higher multiplicities (the contaminants).
==Open-shell wave functions==

Within Hartree–Fock theory, the wave function is approximated as a Slater determinant of spin-orbitals. For an open-shell system, the mean-field approach of Hartree–Fock theory gives rise to different equations for the α and β orbitals. Consequently there are two approaches that can be taken – either to force double occupation of the lowest orbitals by constraining the α and β spatial distributions to be the same (restricted open-shell Hartree–Fock, ROHF) or permit complete variational freedom (unrestricted Hartree–Fock UHF). In general, an ''N''-electron Hartree–Fock wave function composed of ''N''α α-spin orbitals and ''N''β β-spin orbitals can be written as
:\Psi^_\sigma(1)\cdots\mathbf_\sigma(N)) = \mathcal\left(\psi_^(\mathbf_\alpha_)\cdots\psi_(\mathbf_})
\psi_^(\mathbf_\beta_)\cdots\psi_^(\mathbf_\beta_)\right).
where \mathcal is the antisymmetrization operator. This wave function is an eigenfunction of the total spin projection operator, ''Ŝ''z, with eigenvalue (''N''α − ''N''β)/2 (assuming ''N''α ≥ ''N''β). For a ROHF wave function, the first 2''N''β spin-orbitals are forced to have the same spatial distribution:
:\psi^_(\mathbf_) = \psi^_(\mathbf_),\ \ \ 1\leq j\leq N_.
There is no such constraint in an UHF approach.〔


抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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