16.3 Geometry specifications

The geometry may be given in standard Z-matrix form, XYZ form, or cartesian and polar coordinate MOLPRO92 format. The geometry specifications are given in the form

geometry={,
options
atom specifications
}

The following are permitted as options:

Any valid combination of symmetry generators, as described in the previous section.

NOSYM

Disable use of symmetry.

ANGSTROM

Bond lengths specified by numbers, or variables without associated units, are assumed to be in Å.

CHARGE

Orient molecule such that origin is centre of charge, and axes are eigenvectors of quadrupole moment.

MASS

Orient molecule such that origin is centre of mass, and axes are eigenvectors of inertia tensor (default).

NOORIENT

Disable re-orientation of molecule.

ZSIGNX+

Force first non-zero x-coordinate to be positive.
Similarly, ZSIGNY+, ZSIGNZ+ can be set for the y- and z-coordinates, respectively. If - is used instead of + as last character, the corresponding coordinate is forced to be negative. This can be useful to fix the orientation of the molecule across different calculations and geometries. Alternatively, the system variables ZSIGNX, ZSIGNZ, ZSIGNZ can be set to positive or negative values to achieve the same effect.

PLANEXZ

For the $C_{2v}$ and $D_{2h}$ point groups, force the primary plane to be $xz$ instead of the default $yz$. The geometry builder attempts by swapping coordinate axes to place as many atoms as possible in the primary plane, so for the particular case of a planar molecule, this means that all the atoms will lie in the primary plane. The default implements recommendation $5a$ and the first part of recommendation $5b$ specified in J. Chem. Phys. 55, 1997 (1955). PLANEYZ and PLANEXY may also be specified, but note that the latter presently generates an error for $C_{2v}$.