`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 [82].
All calculations employ the resolution-of-the-identity (RI) approximation for
the electron repulsion integrals needed for the correlation treatment and
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 to excited state transitions with the models CCS and CC2
**first-order properties**- for the ground state (SCF/CCS, MP2, and CC2) and excited states (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
**gradients for auxiliary basis sets**- for RI-MP2, -CC2, etc. calculations based on the RI-MP2 error functional
**R12 corrections**- to RI-MP2; MP2 ground-state energies can be computed (in symmetry) using explicitly-correlated two-electron basis functions in the framework of the MP2-R12 model [83].

- 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:
- Fast RI-MP2 calculations with the
`ricc2`program: - Diagnostics:

- Calculation of Excitation Energies

- 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
- RI-MP2-R12 Calculations
- Parallel RI-MP2 and RI-CC2 Calculations
- Spin-component scaling approaches (SCS/SOS)