# Inactive core and virtual orbitals

All the examples given so far in this chapter have assumed that all the electrons are correlated and all the virtual orbitals are used for excitations. It is possible to exclude core orbitals from the correlation process. It is also possible to omit some of the virtual orbitals. This speeds up the calculation. A note of caution however - though the orbital list can be restricted in energy calculations it is not possible to do so in geometry optimisations (discussed in the next chapter).

Here is the formaldehyde example given at the beginning of this chapter, with the addition of one extra keyword.

```TITLE
Formaldehyde 631g* basis MP2 energy
SYMMETRY
CNV 2
END
VARIABLES
RCO 1.21597 A
RCH 1.101978 A
HCO -121.827934 D
END
BASIS 631G*
ATOMS
XAXIS -1 1.0 0.0 0.0
CARBON 6.0 0.0 0.0 0.0
OXYGEN 8.0 0.0 0.0 RCO
H1 1.0 PTC CARBON OXYGEN XAXIS RCH HCO
END
ACTIVE 3 TO 34
MP4
ENERGY
START
FINISH```
In this calculation there are 34 basis functions and therefore 34 molecular orbitals. The first two molecular orbitals are the 1s core orbitals on the carbon and oxygen atoms. The directive

ACTIVE 3 TO 34

indicates which orbitals are to be used in the calculation, ie. the two core orbitals are omitted.

The output from such a calculation looks like this (compare the values with those given in the first section in this chapter).

``` MP2 CORRELATION ENERGY     -0.3033729141
TOTAL MP2 ENERGY         -114.1676787881
MP3 CORRELATION ENERGY     -0.0053276561
TOTAL MP3 ENERGY         -114.1730064443
MP4(SDQ) CORRELATION ENERGY     -0.0086034494
TOTAL MP4(SDQ) ENERGY         -114.1816098936
MP4 CORRELATION ENERGY     -0.0188618859
TOTAL MP4 ENERGY         -114.1918683301
Feenburg MP4 Energy      -114.1928945705```
It would also be possible to omit some virtual orbitals. For example if you choose to omit the two lowest occupied orbitals and the two highest virtual orbitals then the directive would be,

ACTIVE 3 TO 32

for this particular case.

The ACTIVE directive should work in all correlated energy calculations.

It is also possible to use an INACTIVE directive rather than ACTIVE. This may be more convenient is some circumstances. The format is

INACTIVE i j k l ....

where i, j, k, l .... are the labels of those molecular orbitals which one wishes to omit. For example in the above case, the directives

ACTIVE 3 TO 32

and

INACTIVE 1 2 33 34

produce the same results.