Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. 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 and 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, 2, but can formally be expanded in terms of pure spin states of higher multiplicities (the contaminants).