10.5 Ground State Second-order Properties with MP2 and CC2

Second-order properties for one-electron perturbation can be computed at the MP2 and the CC2 level. For MP2, second-order properties are computed as derivatives of the SCF+MP2 total energy. This approach includes the relaxation of the SCF orbitals in the presence of the perturbation and is restricted to the static (i.e. frequency-independent) limit.

For coupled-cluster model CC2, second-order properties can, similar as the first-order properties, calculated in orbital-unrelaxed or orbital-relaxed approach as derivatives of the of the Lagrange functions in Eqs. 10.12 and 10.15. As for MP2, the orbital-relaxed calculations are restricted to the static limit. Frequency-dependent second-order properties as e.g. dipole polarizabilities can be computed with the orbital-unrelaxed approach.

Since V7.2 second-order properties are also available in the MPI parallel version and also for unrestricted high-spin open-shell Hartree-Fock refrences. Note, that second-order properties not available for spin-component scaled variants of MP2 and CC2 or for restricted open-shell references. Furthermore, non-Abelian point groups and point groups with complex irreducible representations are not implemented for second-order properties.

In addition to the standard input, second-order properties require that the data group for the numerical Laplace transformation $laplace and that the sops option in the data group $response is set. Frequency-dependent dipole polarizabilities with the CC2 model are obtained with the input:

  sop operators=(diplen,diplen) freq=0.077d0

The frequency has to be given in atomic units. Static orbital-relaxed polarizabilities are obtained with

  sop operators=(diplen,diplen) relaxed