Ab initio UHF calculations. Part 8.—Spin contamination in the cyanide radical

Abstract

Ab initio UHF calculations on the cyanide radical have been extended to reduce the excessive amount of spin contamination which is sometimes characteristic in the UHF method. The “spin constrained UHF method” minimizes the function αE_UHF+(1–α)〈S²〉_UHF for values of α between 0 and 1, to produce UHF-type wave functions. The coefficients of the contaminating spin states are determined for selected values of α. By an extrapolation procedure the hyperfine coupling constants arising from the doublet-state component of the UHF wave function (α= 1) are estimated to be a(¹³C)= 238 G and a(¹⁴N)=–4.2 G.