Coupledcluster response theory with linearr_{12} corrections: The CC2R12 model for excitation energies
Abstract
Coupledcluster response theory for vertical excitation energies within the secondorder approximate coupledcluster singlesanddoubles model CC2, including linearr_{12} corrections, is derived and implemented for Ansätze 1 and 2 of R12 theory. An orthonormal auxiliary basis set is used for the resolutionoftheidentity approximation in order to calculate the three and fourelectron integrals needed in R12 theory. The basis set convergence is investigated for a selected set of atoms and small molecules and it is found that in many cases the convergence is not improved. An analysis of the different contributions to excitation energies shows that the present scheme for the construction of the R12 pair functions leads in response theory to an unbalanced description of ground and excitedstate wave functions and needs to be generalized to carry the high accuracy of R12 methods over to response theory.
 Publication:

Journal of Chemical Physics
 Pub Date:
 January 2006
 DOI:
 10.1063/1.2161183
 Bibcode:
 2006JChPh.124d4112F
 Keywords:

 31.15.Dv;
 31.25.Jf;
 31.15.Md;
 Coupledcluster theory;
 Electron correlation calculations for atoms and ions: excited states;
 Perturbation theory