Singlet-triplet gaps in diradicals by the spin-flip approach: A benchmark study
نویسندگان
چکیده
The spin-flip approach has been applied to calculate vertical and adiabatic energy separations between low-lying singlet and triplet states in diradicals. The spin-flip model describes both closedand open-shell singlet and ~low-spin! triplet states within a single reference formalism as spin-flipping, e.g., a→b , excitations from a high-spin triplet (M s51) reference state. Since both dynamical and nondynamical correlation effects are much smaller for the high-spin triplet states than for the corresponding singlet states, the spin-flip models yield systematically more accurate results than their traditional ~non-spin-flip! counterparts. For all the diradicals studied in this work, the spin-flip variant of the coupled-cluster model with double excitations yields energy separations which are within less than 3 kcal/mol of the experimental or the highly accurate multireference values. In most cases the errors are about 1 kcal/mol. © 2002 American Institute of Physics. @DOI: 10.1063/1.1498819#
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