ricc2 is a module for the calculation of excitation energies and response properties at a correlated second-order ab initio level, in particular the second-order approximate coupled-cluster model CC2 , but also the MP2, CIS(D), CIS(D∞), and ADC(2) levels. 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 induced transition moments using SOC-PT, 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, two-photon and induced transition moments.) 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) .
As listed above, some functionalities are, as a side-produce, in ricc2 also implemented at the uncorrelated HF-SCF, CIS, and CCS levels. There are only made available in ricc2 for easier test calculations and comparisons, without that the code in ricc2 has optimized for them.
For calculations with CCSD, CCSD(T) and other higher-order models beyond CC2 see Chapter 11.