Controlling spin contamination using constrained density functional theory.

We have extended the constrained density functional theory (DFT) approach to explicitly control the magnitude of spin contamination. Unlike a restricted or restricted open-shell approach, the present method allows finer granularity, not only constraining the magnitude of the spin contamination but also allowing for the possibility of applying the constraint to a subsystem of a much larger system. This allows for the description of spin polarization where physically meaningful, while simultaneously enabling the reduction of spurious overpolarization that is present in many DFT functionals. We utilize this constraint in two particular model applications: The calculation of isotropic and anisotropic hyperfine couplings of a transition metal complex, [Mn(CN)(5)NO](2-), and the calculation of the diabatic dissociation curves of OF radical. In both cases, the spin contamination constraint is essential for obtaining physically meaningful, qualitatively correct, results.

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