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Excitation energies with CCSD:

Since release V6.4 electronic excitation energies can also be computed at the (conventional) CCSD level. For this the data group $excitations has be added (the same keyword as for CC2 apply). The implementation is currently restricted to vertical excitation energies (no transition moments or properties available) and in the closed-shell case to singlett excited states.

Note that for single-excitation dominated transitions CCSD is as CC2 correct through second-order in H and does not neccessarily more accurate than CC2. It is, however, for double excitations still correct through first-order, while CC2 describes double excitations only in a zero-order approximation. Therefore, CCSD results are more robust with respect to double excitation contributions to transitions and are thus usefull to check if CC2 is suitable for a certain problem.


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