EFFECT OF SPIN CONTAMINATION IN UHF WAVE-FUNCTIONS ON CHARGE DENSITY-BASED LOCAL QUANTITIES

The effect of spin contamination in UHF wavefunctions on charge densit y-based local quantities has been investigated for 16 open-shell molec ules. These quantities are found to be quite sensitive to spin contami nation. When the [(S) over cap(2)$]-error in the UHF method is appreci able, the calculated local quantities deviate markedly from the classi cally expected values. The spin contamination can be totally eliminate d either by the ROHF or by the SUHF (spin constrained UHF) method. In the latter the [(S) over cap(2)$]-error is constrained using a Lagrang ian multiplier, lambda. The SUHF solutions gradually converge to the R OHF solutions as lambda --> infinity. However, most of the spin contam ination is removed by using very small positive values of lambda. The lowest value of lambda for which the [(S) over cap(2)$]-error (calcula ted up to the fourth place of decimal) vanishes is referred to as the threshold lambda, or lambda(t). Using lambda = lambda(t) excellent agr eement is obtained between the SUHF and ROHF values for various local quantities.