PM6

[Teodoro Laino]

Dear All,

good news:
with the today commit, (finally !!), there's the possibility to run  
PM6 semi-empirical calculation in
CP2K (fully support for all elements) (at the moment only numerical  
derivatives.. analytical will come soon).

bad news:
In the original version of PM6 there are some terms that make the  
heat of formation a discontinuous function of
the nuclei positions (are just correction terms, constants that  
become zero for different topological configurations) that
we didn't implement. Therefore for few situations there could be  
discrepancies between the heat of formation of CP2K
and the one of MOPAC (but the electronic energy should anyway be the  
same (within numerical  errors (see below)).
An example is H-CC-H, that has a constant correction term to the heat  
of formation of the triple bond that becomes
zero when the triple bond is broken (it's just a step function).

Why have been these terms omitted?
Simply because there's no trace of them in the PM6 paper.

I'm sure that there could be other hidden corrections/parameters not  
explicitly mentioned in the paper (classical correction
like the one for correcting the sp2 pyramidalization, that we didn't  
implement as well), so in case of big discrepancies
compared with MOPAC2007 feel free to post here your input file and a  
short description of the problem.

What does it mean big discrepancies?
These are the values for the heat of formation (kcalmol) of the  
following systems (CP2K/MOPAC2007)

HCl  (17.0356/17.0345)
BrCl (2113.534/2113.493)
TiO (202.714/202.633)

If you have some discrepancy larger than the ones above, check always  
the value of the electronic energy that
must anyway be VERY similar (the only difference is in the way we  
compute integrals.. so just kind of numerical
errors, totally negligible).
Have fun!!

Teo