Explicitly correlated similarity transformed equation-of-motion coupled-cluster method

Similarity transformed equation-of-motion method, based on linearly approximated explicitly correlated coupled-cluster singles and doubles [CCSD(F12)] model, has been formulated and implemented. An extension of similarity transformation operator is introduced in order to treat short-range correlation effects for excited states. Additionally, effective reduction of the number of active virtuals can be obtained by such modification. Numerical tests for sets of valence and Rydberg excited states of several molecules are conducted. Statistical measures of errors in excitation energies show that explicitly correlated results are accurate up to 0.1 e.V already at a double-ζ level compared to those in the complete basis set limit, if the excitation energy is not too close to an ionization threshold. An example of long-range charge transfer excitation is also considered and highly accurate results are obtained.