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 (with the exception of second-order propeties).
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 (cmp. Sec. 8.3).
(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 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
- second-order properties
- for the ground state with MP2 and CC2 and a closed-shell RHF
reference wavefunction (currently restricted to the sequentical and SMP parallel versions)
- 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
- solvent effects
- for the methods and states for which (orbital-relaxed) densities are
available equilibrium solvent effects can be included in the framework of the cosmomode
(for details see Chapter 17).
The second-order models MP2, CIS(D), CIS(D∞
), ADC(2) and CC2 can
be combined with a spin-component scaling (SCS or SOS).
(Not yet available for second-order properties.)
For the SOS variants ground state and excitation energies can be
-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.