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 FINISHIn 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 -114.1928945705It 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
INACTIVE 1 2 33 34
produce the same results.