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

Yao Yuan Chuang, Elena L. Coitiño, Donald G. Truhlar

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85 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)446-450
Number of pages5
JournalJournal of Physical Chemistry A
Volume104
Issue number3
DOIs
StatePublished - Jan 27 2000

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