Normally, the orbital domains are determined automatically using the procedure of Boughton and Pulay, J. Comput. Chem., 14, 736 (1993) and J. Chem. Phys. 104, 6286 (1996). The selection criterion can be modified by the DOMSEL key (see section 22.5. It is also possible to define the domains ``by hand'', using the DOMAIN directive:
DOMAIN,orbital,atom1, atom2 ...
where orbital has the form iorb.isym, e.g., 3.1 for the third orbital in symmetry 1, and atomi are the atomic labels as given in the Z-matrix geometry input, or, alternatively, the Z-matrix row numbers. All basis functions centred at the given atoms are included into the domain. For instance
DOMAIN,3.1,C1,C2
defines a domain for a bicentric bond between the carbon atoms C1 and C2. The DOMAIN cards must be given after any OCC, CLOSED, or CORE directives. Note that the order of the localized orbitals depends on the localization procedure, and could even change as function of geometry, and therefore manual DOMAIN input should be used with great care. The domains of all orbitals which are not explicitly defined using DOMAIN cards are determined automatically as usual.
P.J. Knowles and H.-J. Werner