This keyword specifies the uses of molecular symmetry within the calculation. By default, the program attempts to identify the point group of the molecule. 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. Input for properties and background charge distributions must be specified in the standard orientation.
By default, symmetry is used wherever possible to reduce CPU, disk storage, and I/O requirements. The NoSymmetry keyword prevents molecule reorientation and causes all computations to be performed in the input orientation (although the program still attempts to identify the appropriate point group). Symmetry use can be completely disabled by Symmetry=None; use this option if NoSymm generates an error when identifying the point group.
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. Symm=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!
CenterOfCharge
Use the center of atomic charges in determining the standard orientation. This is the default. COC is a synonym for this option.
CenterOfMass
Use the center of atomic masses in determining the standard orientation. COM is a synonym for this option.
None
Do not assign point group and bypass all symmetry processing.
Last update: 23 April 2013
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