Effect of spin contamination in UHF wavefunctions on charge density-based local quantities

Abstract

The effect of spin contamination in UHF wavefunctions on charge density-based local quantities has been investigated for 16 open-shell molecules. These quantities are found to be quite sensitive to spin contamination. When the 〈Ŝ2〉-error in the UHF method is appreciable, the calculated local quantities deviate markedly from the classically expected values. The spin contamination can be totally eliminated either by the ROHF or by the SUHF (spin constrained UHF) method. In the latter the 〈Ŝ2〉-error is constrained using a Lagrangian multiplier, λ. The SUHF solutions gradually converge to the ROHF solutions as λ → ∞. However, most of the spin contamination is removed by using very small positive values of λ. The lowest value of λ for which the 〈Ŝ2〉-error (calculated up to the fourth place of decimal) vanishes is referred to as the threshold λ, or λt. Using λ = λt excellent agreement is obtained between the SUHF and ROHF values for various local quantities.