29.6.1 Standard domains

Standard domains are always determined first. They are used to define strong, close, weak, and distant pairs. More accurate results can be obtained with extended domains, as described in section 29.6.2.

THRBP=value

Threshold for selecting the atoms contributing to orbital domains using the method of Boughton and Pulay (BP). As many atoms as needed to fulfill the BP criterion are included in a domain. The order in which atoms are considered depends on the parameter MAXBP, see below. The default is THRBP=0.98. THRBP=1.0 includes all atoms into each orbital domain, i.e., leads to full domains. If no pairs are neglected, this should yield the canonical MP2 energy.

The criterion is somewhat basis dependent. See section 29.9.4 for recommended values of this threshold.

CHGMIN=value

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

CHGMINH=value

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

CHGMAX=value

If the atomic 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 the method to determine atomic charges. MULLIKEN=0 (default): squares of diagonal elements of ${\bf S}^{\frac{1}{2}} {\bf C}$ are used (Löwdin charges); MULLIKEN=1: Mulliken gross charges are used. The first choice is less basis set dependent and more reliable with diffuse basis sets.

MERGEDOM=number

If number is greater than zero, 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 29.9.7 for further discussion of geometry optimizations.

There are some other options which should normally not be modified:

DELBAS=ibaso

This parameter determines the method for eliminating redundant functions of pair domains.
ibaso=0: The space of normalized eigenvectors of ${\bf\tilde S}^{ij}$, which correspond to small eigenvalues, is eliminated (default). Any other value is not recommended and not further documented.

DELCOR=nshell

Activates elimination of basis functions corresponding to core orbitals. If nshell=1, only $1s$-functions are eliminated from projected space. If nshell=2 (default) $1s$ functions on first-row atoms, and $1s$, $2s$, and $2p$-functions are eliminated on second-row atoms. Nothing is eliminated on H or He atoms. If effective core potentials are used, nothing is deleted at the corresponding atom. Also, functions are only deleted if the norm of the projected function is below THRCOR (default 0.1)