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Second-Order Approximate Coupled-Cluster (CC2) Calculations


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
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 C1 symmetry) using explicitly-correlated two-electron basis functions in the framework of the MP2-F12 model [110,106].
All functionalities at the MP2 and CC2 level are implemented for closed-shell RHF and open-shell UHF reference wavefunctions. Calculations with the method CIS, CIS(D), CIS(D) and ADC(2) are presently restricted to closed-shell RHF and open-shell UHF reference wavefunctions. 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. (Note, that presently no gradients are available for MP2 and CC2 with ROHF reference wavefunctions.)

The second-order models MP2, CIS(D), CIS(D), ADC(2) and CC2 can be combined with a spin-component scaling (SCS or SOS). For the SOS variants ground state and excitation energies can be computed with $ \cal {O}$($ \cal {N}$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.

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