How should we calculate transition state geometries for radical reactions? The effect of spin contamination on the prediction of geometries for open-shell saddle points

In this article we systematically examine the ability of current electronic structure methods to treat transition states whose unrestricted wave functions show significant spin contamination. Three H atom abstraction reactions have been selected as test cases for the study, namely the reactions of trans-N₂H₂ with H, CH₄ with OH, and C₂H₆ with OH. In each case we calculate the exoergicity, barrier heights, and transition state geometry at 3 to 26 levels of theory. The spin contamination in spin-unrestricted electronic structure calculations of the transition states is in the range of 0.755-0.9. Twelve different kinds of ab initio calculation with electron correlation (UMP2, ROMP2, UMP4, UCCD, UQCISD, UCCSD, UQCISD(T), UCCSD(T), RUCCSD, RCCSD, RUCCSD(T), and RCCSD(T)) are applied with two correlation-consistent basis sets (cc-pVDZ and cc-pVTZ). We conclude that quadratic configuration interaction and coupled cluster methods, even with unrestricted reference states, provide good approximations to transition state geometries and energies.