In unrestricted Hartree–Fock method, the wavefunctions obtained are eigenfunctions of the Hamiltonian and the spin projection S z operators, but not eigenfunctions of the S 2 operator. As a result, the wavefunctions of the doublet systems are spin-contaminated to some extent by admixtures of quartet, sextet, and higher spin states. The eigenvalues of the S 2 operator are given as a measure of the spin contamination.