The CC2 model is an approximated coupled-cluster singles-and-doubles (CCSD) method which has been proposed in 1996 by Christiansen, Jørgensen and Koch for response calculations on molecules which are out of reach for CCSD and higher correlated methods. It is one of the simplest correlated ab initio methods for excited states and yields energies for singly-excited states which are correct through second-order in the electron-electron interaction (dynamic electron correlation), as the well-known second-order Møller-Plesset perturbation theory (MP2) does for ground states. It is thus well-suited for the study of excited states of large closed-shell (or at least "single-reference") molecules. In difference to the perturbative doubles corrections CIS(D) to the widely used configuration interaction singles (CIS) method and similar perturbative approaches to excited states which use non-degenerate perturbation theory, CC2 is not limited to energetically isolated states. A feature, which is important in the search of excited state equilibrium structures. In several applications CC2 has been shown to be a viable tool for such studies.

As MP2 and other related methods based on a second-order treatment of electron correlation, CC2 can be implemented very efficiently with a so-called resolution of the identity approximation for the integrals which describe the electron-electron interaction and thereby made applicable to relatively large molecules, which have been intractable with conventional implementations. As demonstrated in the mid 1990's by Weigend and Häser for MP2, the computationally costs and demands (CPU time, memory and disk space) are for most applications reduced by orders of magnitudes.

During the last years we have developed the ricc2 code of the Turbomole package, an implementation of CC2 with the resolution-of-the-identity approximation which includes

• ground and excitation energies

• transition matrix elements (e.g. oscillator or rotatory strength) and first-order properties (expectation values) of excited states

• analytic gradients for ground and excited states

• excitation energies and gradients for CIS(D_∞) and ADC(2)

• a distributed memory parallel implementation based on the MPI standard

• spin-component scaled SOS and SCS variants

• an implementation of the SOS variants of MP2, CC2 and ADC(2) with O(N⁴) scaling computational costs for ground and excitation energies and gradients, transitions moments and first-order properties

• two-photon transition moments

• static and frequency-dependent polarizabilities for ground states and electronically excited states

• an embedding in an polarizable environment for the prediction of solvent and other environment effects of excited states and spectra including two-photon spectra

• analytic molecular Hessians and dipole gradients for vibrational frequencies and IR intensities

• optical rotation strengths

• oscillator strengths for triplet excited states induced by spin-orbit coupling to compute phosphorescence lifetimes

As a side product, the code includes a revised implementation of RI-MP2 for ground state energies and gradients and implementations of RI-CIS(D) and RI-ADC(2) (algebraic diagrammatic construction through second order, J. Schirmer 1981) for excitation energies. All functionalities at the MP2, CC2, CIS(D) and ADC(2) levale are implemented for closed-shell and unrestricted Hartree-Fock references and most of them are parallelized for PC clusters using the Message Passing Interface (MPI) standard.