Gaussian 03 Online Manual
Last update: 2 October 2006

Symmetry

This keyword specifies the uses of molecular symmetry within the calculation. If symmetry is in use, the molecule may be rotated to a different coordinate system, called the standard orientation, before the calculation is performed. Derivatives are then rotated back to the original (input) orientation. Orbitals are printed in the standard orientation, and input for properties and background charge distributions is required in the standard orientation.

The NoSymmetry keyword prevents the reorientation and causes all computations to be performed in the Z-matrix orientation. By default, symmetry is used wherever possible to reduce CPU, disk storage, and I/O requirements. Symmetry use can be completely disabled by NoSymm, or modified by the Symm keyword and one or more options.

Int
Int enables and NoInt disables use of integral symmetry (use of the "petite list"). Synonymous with Int=[No]Symm.

Grad
NoGrad disables and Grad enables use of symmetry in integral derivative evaluation.

SCF
NoSCF disables and SCF enables use of N³ symmetry in SCF, which is used by default only for GVB calculations. SCF=NoSCF is equivalent to Guess=LowSym and combining all irreducible representations together.

Loose
Tells the program to use looser cutoffs in determining symmetry at the first point. It is designed for use with suboptimal input geometries. Tight says to use the regular criteria at the first point, and it is the default.

Follow
Try to follow point group/orientation during optimization.

PG=group
Use no more symmetry than that found in the specified point group.

Axis=[X|Y|Z]
Specify axis to help specify subgroup.

On
Turn on symmetry when it would otherwise be off, such as with massage. This can cause wrong answers, so it should only be used if you know what you're doing!

Int, SCF