In large molecules, it may be sufficient to correlate only the electrons in the vicinity of an active group, and to treat the rest of the molecule only at the SCF level. The ATOMLIST directive allows the specification of a subset of atoms:
ATOMLIST,atom1, atom2 ...
The program will then correlate only electrons in orbitals whose domains are exclusively covered by the given atoms. Electrons in a bonding orbital from one of the given atoms to one which is not part of the list are not correlated. This may significantly reduce the computation time, and, provided the active atoms are sensibly chosen, may give still sufficiently accurate results for the active group, e.g. bond lengths and bond angles.