29.6.2 Extended domains

There are two alternative modes for domain extensions: either distance criteria REXT, REXTS, REXTC, or REXTW can be used. These are in Bohr and refer to the minimum distance between any atom in a standard orbital domain $[ij]$ and another atom. If an atom is found within the given distance, all PAOs at this atom are added to the domain $[ij]$. Alternatively, connectivity criteria IEXT, IEXTS, IEXTC, or IEXTW can be used. These refer to the number of bonds between any atom contained in the standard domain $[ij]$ and another atom. The advantage of distance criteria is that they select also atoms within the given radius which are not connected to the present domain by bonds. On the other hand, the connectivity criteria are independent of different bond lengths, e.g., for first and second-row atoms. Only one of the two possibilities can be used, i.e., they are mutually exclusive.

REXT=value

Distance criterion for extension of all pair domains.

REXTS=value

Distance criterion for extension of strong pair domains.

REXTC=value

Distance criterion for extension of strong and close pair domains.

REXTW=value

Distance criterion for extension of strong, close, and weak pair domains.

IEXT=value

Connectivity criterion for extension of all pair domains.

IEXTS=value

Connectivity criterion for extension of strong pair domains.

IEXTC=value

Connectivity criterion for extension of strong and close pair domains.

IEXTW=value

Connectivity criterion for extension of strong, close, and weak pair domains.

By default, domains are not extended, i.e., the default values of all parameters listed above are zero. Note that the pair classes are determined on the basis of the standard domains, and therefore domain extensions have no effect on the pair lists.

Also note that the computational effort increases with the fourth power of the domain sizes and can therefore increase quite dramatically when extending domains. This does not affect the linear scaling behaviour in the asymptotic limit.