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 [101]. 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--excited and excited--excited state transitions for 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

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 [102,98].

All functionalities are implemented for 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.)