*Orbital relaxed vs orbital unrelaxed calculations*

As the calculation of frequency-dependent properties is affected by the pole-structure of the
corresponding quantum chemical model, coupled-cluster calculations of these properties should
be always carried without consideration of orbital relaxation (keyword: **DIFF_TYPE=UNRELAXED**), as in this way detoriation of
the results due to the presence of the HF poles is avoided. Note that in this way, corresponding
calculations are **only possible** at **CC2**, **CCSD**, **CC3** levels and, for example, **not** at the popular
**CCSD(T)** level. Note also that calculation at the **MPn** levels are also **not** available.

*Large-scale calculations*

**Large scale calculations** should be performed, if possible, using **partial AO-algorithms**
(keyword: **ABCDTYPE=AOBASIS**, available for **CCSD** but not for **CC3**) and the module
*xecc* (keyword: **CC_PROG=ECC**).

*Frozen-core calculations*

**Frozen-core orbitals** can be used in all CC calculations of frequency-dependent polarizabilities

*CC2 model*

There exist the possibility to perform the corresponding **CC2** calculations with CFOUR, but please note
that CFOUR does not provide an efficient implementation for this CC model.

*Frequency input*

Currently, it is only possible to perform the calculation for **one** frequency.

*File DPOLAR*

The calculated **frequency-dependent polarizability tensor** can be also found on the file **DPOLAR**