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

被引:87
作者
Chuang, YY
Coitiño, EL
Truhlar, DG
机构
[1] Univ Minnesota, Dept Chem, Minneapolis, MN 55455 USA
[2] Univ Minnesota, Inst Superconduct, Minneapolis, MN 55455 USA
关键词
D O I
10.1021/jp993661v
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
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.
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页码:446 / 450
页数:5
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