By default, calculations with CFOUR are carried with **all electrons** and **all orbitals** correlated. **Frozen-core** calculations (with valence electrons correlation alone)
can be performed with

**FROZEN_CORE=ON**

or by explicitly specifying the orbitals to be dropped from the correlation treatment via

**DROPMO=n1-n2 ...n3>n4...**

The numbers supplied with the **DROPMO** keyword (separated by a dash) specify the orbitals to be
dropped, whereby n3>n4 means that all orbitals from n3 through n4 inclusive are dropped. Note
that the orbitals are numbered in **ascending** order from the most stable to the most unstable.

For **UHF**-type calculations, the appropriate orbitals (which are not identical) are deleted for both
spin cases.

In **ROHF**-type calculations, the definition of the frozen-core orbitals depend on whether **standard** or
**semi-canonical** orbitals are used, as the transformation to the semi-canonical representation is
carried out with the full set of orbitals (this for example is done differently in MOLPRO, where
just the correlated orbitals are transformed to a semi-canonical representation; therefore, there
are slight differences in ROHF-CCSD(T) energies obtained with CFOUR and MOLPRO when freezing orbitals).

**Dropped** orbitals are allowed for **energy** as well as **analytic first** and
**second derivative** calculations, though the implementation for analytic
derivative is currently not the most efficient.

General Remarks

Correlated calculations are often performed with only a subset of the
molecular orbitals included in the correlation treatment.

Common choices are:

*all-electron (ae) calculations* involving all electrons and all

(occupied as well as virtual) orbitals in the correlation treatment;

*frozen-core (fc) calculations* considering only the valence

electrons in the correlated calculations in the sense that only the
occupied valence orbitals together with all virtual orbitals are included
in the correlation treatment.

Besides these two common choices, any other choice in principle is also
possible, though often less meaningful.

**Calculations**

The *default* is to perform *all-electron (ae) calculations*, while with the keyword

**FROZEN_CORE=ON**

a standard *frozen-core (fc) calculation* is invoked (note that some care is required
for heavier elements, alkaline as well as alkaline earth elements
concerning the appropriate definition of the core orbitals).

**Standard frozen cores**

Using the **FROZEN_CORE=ON** keyword, the following definitions for the core are applied:

H, HE: no core orbitals

Li-Ne: 1 core orbital

Na-Ar: 5 core orbitals

K-Zn: 9 core orbitals

Ga-Kr: 14 core orbitals

Rb-Cd: 18 core orbitals

In-Xe: 23 core orbitals

Default: **FROZEN_CORE=OFF**

**Recommendation**

In general, frozen-core calculations should be performed
with so-called valence basis sets, while all-electron calculations necessitate
the use of so-called core-polarized basis sets. In the case of the correlation-consistent basis-sets of Dunning, the cc-pVXZ sets are well suited for fc calculations, while the cc-pCVXZ sets are the appropriate choice for ae calculations.

**Implementation**

The information about the *dropped molecular orbitals* is stored on the
following JOBARC records. Record *NUMDROPA* contains the total number of
dropped orbitals, while the records *MODROPA* and *MODROPB*
contain the indices of the corresponding alpha and beta molecular
orbitals to be dropped (ordered in ascending order according to the
orbital energies).