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10.1 Geometry specifications

The geometry may be given in standard Z-matrix form, or XYZ form. The geometry specifications are given in the form

[SYMMETRY, options ]

[ORIENT, options ]

[ANGSTROM]

GEOMETRY={
atom specifications
}

GEOMETRY must come after the other commands that modify the way the geometry is constructed. The following are permitted as SYMMETRY options:

Any valid combination of symmetry generators, as described in section 10.2 below.

NOSYM

Disable use of symmetry.

The following are permitted as ORIENT options:

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 for Z-matrix input). Alternatively, the symmetry centre can be specified as CENTRE=MASS|CHARGE.

NOORIENT

Disable re-orientation of molecule (default for XYZ-input).

SIGNX=$\pm$1

Force first non-zero $x$-coordinate to be positive or negative, respectively. Similarly, SIGNY, SIGNZ can be set for the $y$- and $z$-coordinates, respectively. 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 $C_{2v}$.

ANGSTROM Forces bond lengths that are specified by numbers, or variables without associated units, to use the values as a number of Ångstrom, rather than Bohr.

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