Subject: Symmetry in MOPAC

From: [email-masked] (Dr. James Stewart)
Date: Tue, 8 Aug 95 08:05:33 PDT

Joachim Schulte writes:

> With Mopac 7.0 I have some problems
> concerning the recognition of some point groups. Provided with an 
> icosahedral C60 geometry MOPAC does not recognize any point group, 
> not even a subgroup of Ih. The output simply is '????'. The degeneracy
> scheme of the molecular orbital energies is looking correct (3-, 4- and 
> fivefold degeneracies). This happens both for cartesian coordinates 
> and Z-matrix coordinates (Mopac and Gaussian style). 
> But also with simpler molecules there seem to be problems, e.g. 
> cyclopentadienyl-anion. 
> Mopac returns C5H in that case.  Other molecules, as benzene or 
> methane are correctly(D6H and TD). 
> These problems seem to be hardware-independent (Tried on a Pentium
> running Linux and a DECstation running Ultrix).   

MOPAC 7 was not designed to recognize C60.  MOPAC 7 was limited
to systems for which the principal axis is either perpendiculer
to a set of atoms (Benzene) or goes through some of the atoms.
In C60, the principal axis is through the middle of a pentagon.

For routine use, MOPAC 93 is recommended over MOPAC 7.  MOPAC 93
will recognize C60 as being Ih, and a whole slew of weird and
wonderful systems - truncated icosahedra (like C60), truncated
cubes and tetrahedra, and `twisted' icosahedra (point-group `I')

At the risk of sounding repetitive: Users of MOPAC are encouraged
to use MOPAC 93, and are advised not to use MOPAC 7.  MOPAC 7 is
intended to be used only when a public domain code MUST be used.
MOPAC 7 has a large number of known minor faults - such as the
inability to recognize the symmetry of many systems.  This makes
it particularly irritating, compared to MOPAC 93, when it is used
for routine work.

MOPAC 7 and MOPAC 93 are both available through the QCPE.

Jimmy Stewart