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 .
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:
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.)
- 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 transition moments for ground state--excited state transitions
- 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