`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 [109].
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:

**ground state energies**- for MP2 and CC2 and spin-component scaled
variants thereof; the MP2 results are identical
with those obtained with
`rimp2`(but usually the calculations are somewhat faster). **excitation energies**- for the models CIS/CCS, CIS(D), CIS(D
_{∞}), ADC(2), and CC2 **transition moments**- for ground state--excited and excited--excited state transitions for the models CCS and CC2; for ADC(2) only moments for ground state--excited state transitions are available
**first-order properties**- for the ground state with SCF (CCS), MP2, and CC2
and for excited states with CCS, CC2, ADC(2) and CIS(D
_{∞}) **geometric gradients**- for the electronic ground state at the MP2 and
the CC2 level; for electronically excited states at the CIS(D
_{∞}), ADC(2), and CC2 level **second-order properties**- for the ground state with MP2 and CC2 and a closed-shell RHF reference wavefunction (currently restricted to the sequentical and SMP parallel versions)
**gradients for auxiliary basis sets**- for RI-MP2, -CC2, etc. calculations based on the RI-MP2 error functional
**F12 corrections**- to RI-MP2;
MP2 ground-state energies can be computed (in
*C*_{1}symmetry) using explicitly-correlated two-electron basis functions in the framework of the MP2-F12 model [110,106]. **solvent effects**- for the methods and states for which (orbital-relaxed) densities are
available equilibrium solvent effects can be included in the framework of the
`cosmo`mode (for details see Chapter 17).

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 ground state and excitation energies can be
computed with
(^{4})
-scaling costs if the
Laplace transformation (LT) (keyword `$Laplace` is enabled.

For calculations with CCSD, CCSD(T) and other higher-order models beyond CC2 see Chapter 10.

- Prerequisites
- How To Perform a Calculation
- How to quote
- CC2 Ground-State Energy Calculations
- Advantages of the RI approximation:
- Required input data:
- Ground-State calculations for other methods than CC2:
- Diagnostics:

- Calculation of Excitation Energies
- Running excitation energy calculations:
- Trouble shooting:
- Diagnostics for double excitations:
- Visualization of excitations:

- First-Order Properties and Gradients
- Ground State Properties, Gradients and Geometries
- Excited State Properties, Gradients and Geometries
- Visualization of densities and Density analysis
- Fast geometry optimizations with RI-SCF based gradients

- Transition Moments

- Ground State Second-order Properties with MP2 and CC2
- Parallel RI-MP2 and RI-CC2 Calculations
- Spin-component scaling approaches (SCS/SOS)

- Polarizable embedding calculations