ricc2 is a module for the calculation of excitation energies and response properties at a correlated ab initio level, in particular the second-order approximate coupled-cluster model CC2 . All calculations employ the resolution-of-the-identity (RI) approximation for the electron repulsion integrals used in the correlation treatment and the description of excitation processes. At present the following functionalities are implemented:
All functionalities at the MP2 and CC2 level are implemented for closed-shell RHF and open-shell UHF reference wavefunctions (with the exception of second-order propeties, which are only available for a closed-shell RHF reference). Ground state energies for MP2, MP2-F12 and CC2 and excited state energies for CC2 are also implemented for single determinant restricted open-shell Hartree-Fock (ROHF) reference wavefunctions (cmp. Sec. 9.3). (Note, that no gradients are available for MP2 and CC2 with ROHF reference wavefunctions.) For a two-component GHF reference wavefunction energies for the CCS, MP2/ADC(2), CIS(D∞) and CC2 methods as well as ground state—excited state transtition moments for ADC(2) and CC2 are available.
The second-order models MP2, CIS(D), CIS(D∞), ADC(2) and CC2 can be combined with a spin-component scaling (SCS or SOS). (Not yet available for second-order properties.) For the SOS variants one can switch to an implementation with (4)-scaling costs by setting the keyworkd for the numerical Laplace transformation (LT) ($laplace) .
For calculations with CCSD, CCSD(T) and other higher-order models beyond CC2 see Chapter 11.