Specification of molecular symmetry and basis sets¶
This section is needed for calculations using the simple .xyz input format for the molecular structure. For calculations with the .mol input format, basis set information is to be specified together with the atomic coordinates.
This section should contain the keywords needed to specify the basis set. The minimum specification is a default basis set for all atoms in the system.
The name of the basis set file that is to be searched for basis sets.
Example: use the double zeta basis sets optimized by Dyall:
Special basis sets for specific atom types.
The simplest is to specify an alternative basis set library file. One then lists the atomtype, the keyword BASIS, and the name of the alternate file.
Example: use Dunnings cc-pVDZ basis for the fluorine atom:
.SPECIAL F BASIS cc-pVDZ
Basis sets may also be explicitly typed using the EXPLICIT keyword and the format that is also used in the .mol file.
Example: specification of a basis set for the atomtype HB:
.SPECIAL HB EXPLICIT 2 1 1 4 0 3 13.0100000 1.9620000 0.4446000 0.1220000 1 0 3 0.7270000
Note that you may use different basis sets for the same element, in the example above you may have most hydrogen atoms labeled as H and have a few special ones labeled as HB.
We can also introduce point charges that have no basis set attached by specifying the keyword NOBASIS after the atomtype.
Example: a point charge at the position of an oxygen:
.SPECIAL O.PC NOBASIS
The keyword .SPECIAL can be repeated as often as you like, atomtypes not listed as special will receive the default basis set.
Example: EuF3 with dyall set on Eu and the cc-pVDZ on the fluorides:
**MOLECULE *BASIS .DEFAULT dyall.cv2z .SPECIAL F BASIS cc-pVDZ
This optional section allows modification of the properties of a center. This makes it possible to specify fractional charges (for embedding point charges) or zero charge (for ghost centers ised in counterpoise calculations).
The atom type followed by the modified nuclear charge.
Example: a fractional charge of -0.6 for an oxygen point charge:
.NUCLEUS O.PC -0.6
In case the center with the modified charge should contain a basis set there is a second keyword to specify the real nuclear charge that is to be used when searching the basis set file for the element-specific basis set.
Example: an oxygen ghost center (zero charge):
.NUCLEUS O.Gh 0.0 8.0
Like the .SPECIAL keyword described above also the .NUCLEUS keyword can be specified as often as necessary. Note that all atomtypes should have exactly the same name as they appear in the xyz file, it is thereby good practice to use a logical naming scheme like the one used above (with a suffix Gh to indicate ghost atoms and a suffix pc to indicate point charges) but this is not enforced by the program.