Chem. Phys. Lett. 328, 291-301 (2000).
(Received 25 April 2000; received in final form 18 July 2000)
We report an atomic integral direct implementation of the approximate coupled cluster singles and doubles method CC2 for the triplet excitation energies of closed shell molecules. Employing an explicitly spin-coupled triplet excitation space we obtain a compact and efficient formulation which needs approximately the same operation count as required for singlet excitation energies. Compared to a spin orbital based approach this gives a reduction by a factor of 3. A basis set study employing basis sets with up to 432 functions is presented for the lowest excitations in benzene and the results are compared with other theoretical and experimental results.
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