The extended explicitly-correlated second-order approximate coupled-cluster singles and doubles ansatz suitable for response theory.

We report the extended explicitly correlated approximate coupled-cluster singles and doubles CC2(F12^*)-XSP method suitable for response properties. Equations are derived using an automated approach and have subsequently been hand-coded into the computer program KOALA, in which for all two-electron integrals, density fitting is employed. Numerical results are presented for the lowest two vertical singlet excitation energies of a set of selected molecules. The results show that the CC2(F12^*)-XSP method provides the correct basis-set limit with no bias to the ground state, and an excellent agreement with reference CC2 values using large basis sets is found. Using Dunning's aug-cc-pVTZ basis, the CC2(F12^*)-XSP method yields excitation energies which are converged within 1 mE_h to the basis-set limit for valence excitations. [ABSTRACT FROM AUTHOR]