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 at the SCF level. This approach can even be extended, different correlation levels may be used for different sections of the system. The REGION directive allows the specification of a subset of atoms:
REGION,METHOD=method,[DEFAULT=default_method], [TYPE=INCLUSIVEEXCLUSIVE], atom1, atom2 ...
The orbitals located at these atoms will be treated at the level specified in method. The remaining orbitals will be treated as defined in default. If not given by the user, the latter option will be set to HF.
The orbital selection can be done in two ways. If type is set to INCLUSIVE, any orbital containing one of the atoms in its domain centre list will be included. If type is set to EXCLUSIVE, the program will only add orbitals whose domains are exclusively covered by the given atoms. Any local correlation treatment can be given as method, with the restriction that only MP2 and HF can be used as default_method. Up to two REGION directives may be included in a single calculation, ordered according to the correlation level (method) specified for the region. The highest level region should be given last.
It is advisable to check the region orbital list and the orbital domains printed by the program. The use of regions 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.