Comparison of Faults in PM6

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Several severe faults have been reported in PM6. During the development of PM7, an effort was made to correct or remove these faults. 

Faults corrected or removed: 

* A large number of spurious non-bonding interactions have been removed..
* The Si-O-H angle is nearer to that expected, i.e., not 180 degrees.
* Relative energies of Zwitterions and neutral species is improved.  The fault reported in PM6 was less important than originally thought.  Thus in the hydroxy-anilines, the calculated heat of sublimation is similar to that expected.
* The point-group for Ferrocene is now D5d, not C2v as in PM6
* [FeIIIF6]3- is predicted to have Oh symmetry.
* Oxalic acid is planar, when the H-O-C=O are trans (180 degrees) not cis (zero degrees) .  In gas phase, the cis form is predicted to be non-planar, but in the crystal, it is predicted to be planar.  Therefore, the non-planarity of the isolated molecule does not seem to be important.
* The amine - carboxylic acid system is now more stable than the tetrahedral intermediate.
* BrF3 is now more stable, by 3.0 kcal/mol, in C2v symmetry than as D3h.
* BrF5 is now more stable, by 15.1 kcal/mol, in C4v symmetry than as D3h.

Faults not corrected:

* Fe(CO)5 is still C4v (This is being worked on)
* Biphenyl has a torsion angle of 58.5 degrees, with a barrier at 90 degrees of 0.15 kcal/mol. For the same system, PM6 gives the same torsion angle, and a barrier of 0.20 kcal/mol.
* PM7 N-(Benzylidene)aniline is even more stable as cis than as trans, 2.94 kcal/mol in PM7 versus 1.85 kcal/mol in PM6.