26.3.1 Always...

Turn off symmetry! Otherwise, you won't get appropriately localized orbitals (local orbitals will tend to be symmetry equivalent instead of symmetry adapted). Symmetry is in principle OK only if all atoms are symmetry unique. This allows the treatment of planar molecules in $C_s$ symmetry when using the LOCAL directive. But note that the multipole program does not support symmetry at all, so choose always $C_1$ symmetry with the MULTP directive.

To turn off symmetry, specify nosym as the first line of your geometry input, e.g.

geometry={
  nosym
  O1
  H1,O1,roh
  H2,O1,roh,h1,hoh
}

Use NOORIENT! Under certain circumstances it may happen that the domains and correlation energies are not rotationally invariant. We therefore recommend to use the NOORIENT option in the geometry input, to avoid unintended rotations of the molecule when the geometry changes. This is particularly important for geometry optimizations and for calculations of interaction energies (see section 22.3.3).

Check your orbital domains! Local correlation methods are less `black box' than the canonical ones. It is therefore recommended always to check the orbital domains, which are printed in the beginning of each local calculation. For checking, the option DOMONLY=1 can be used to stop the calculation after the domain generation. The orbital domains consist of all basis functions for a subset of atoms. These atoms are selected so that the domain spans the corresponding localized orbital with a preset accuracy (alterable with key DOMSEL). A typical domain output, here for water, looks like this:

 Orbital domains

   Orb.   Atom    Charge       Crit.
   2.1    1 O1     1.17        0.00
          3 H2     0.84        1.00
   3.1    1 O1     2.02        1.00
   4.1    1 O1     1.96        1.00
   5.1    1 O1     1.17        0.00
          2 H1     0.84        1.00

This tells you that the domains for orbitals 2.1 and 5.1 comprise the basis functions of the oxygen atom and and one hydrogen atom, while the domains for orbitals 3.1 and 4.1 consist of the basis function on oxygen only. The latter ones correspond to the oxygen lone pairs, the former to the two OH bonds, and so this is exactly what one would expect.

Improper domains could result from improperly localized orbitals or forgotten NOSYM directive. This does not only negatively affect performance and memory requirements, but can also lead to bogus results. Poor localization is sometimes an intrinsic problem, in particular for strongly conjugated systems. In rare cases it might also happen that the localization procedure does not converge.

The default for the selection criterion DOMSEL is 0.98. This works usually well for small basis sets like cc-pVDZ. For larger basis sets like cc-pVTZ we recommend to use a slightly larger value of 0.985 to ensure that enough atoms are included in each domain.

There are some other options which affect the domain selection:

CHGMIN=value

determines the minimum allowed Mulliken charge for an atom (except H) in a domain, i.e., atoms with a smaller (absolute) charge are not included, even if the DOMSEL criterion is not fulfilled (default 0.01).

CHGMINH=value

as CHGMIN, but used for H-atoms (default 0.03).

CHGMAX=value

If Mulliken charge is larger than this value, the atom is included independent of any ranking.

MAXBP=maxbp

If maxbp=1, the atoms are ranked according to their contribution to the Boughton-Pulay overlap. If maxbp=0 (default), the atoms are ranked according to atomic charges. In both cases atoms with charges greater than CHGMAX are always included, and atoms with the same charges are added as groups.

MULLIKEN=option

Determines method to determine atomic charges. option=0 (default): squares of diagonal elements of ${\bf S}^{\frac{1}{2}} {\bf C}$ are used. option=1: Mulliken gross charges. It appears that the first choice works better with diffuse basis sets.

MERGEDOM=number

If this option is given, all orbital domains containing number or more atoms in common are merged (number=1 is treated as number=2, default 0). This is particularly useful for geometry optimizations of conjugated or aromatic systems like, e.g., benzene. In the latter case, MERGEDOM=1 causes the generation of full $\pi$-domains, i.e., the domains for all three $\pi$-orbitals comprise all carbon basis functions. Note that the merged domains are generated after the above print of orbital domains, and information about merged domains is printed separately. See section 22.3.4 for further discussion of geometry optimizations.

These options can be disabled by setting their values to zero.