Second-Order Approximate Coupled-Cluster (CC2) Calculations

RI-CC2

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; 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$_\infty$ mathend000#), ADC(2), and CC2

transition moments

for ground state--excited state transition and the models CCS and CC2

first-order properties

for the ground state (SCF/CCS, MP2, and CC2) and excited states (CCS and CC2)

geometric gradients

for the electronic ground state at the MP2 and the CC2 level; for electronically excited states at the CIS(D$_\infty$ mathend000#), 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 $C_1$ mathend000# symmetry) using explicitly-correlated two-electron basis functions in the framework of the MP2-R12 model [83].

All functionalities are implemented for closed-shell RHF and open-shell UHF reference wavefunctions.