[CP2K:1742] Re: DFTB charge equilibration discrepancy

Juerg Hutter hut... at pci.uzh.ch
Sat Jan 31 15:21:09 UTC 2009

> I don't think an odd number of electrons is causing the problem.  For
> example, if I add a -1 charge to NO, the codes still disagree.
> However, contrary to what I wrote earlier, DFTB+  and CP2K do agree
> for CO and NH3.  They still disagree for CO2, NO2, and for systems
> such as nitromethane and TATB.  As a wrote earlier, I get excellent
> comparisons for H2O, diamond, methane, and LN2.  It's just for CO2 and
> systems with NO2 that I observe a discrepancy.  The energies for these
> systems tend to be off by ~0.6 Ha, and the forces are off by anywhere
> from 10-100%.  I also tried turning off the scc charge equilibration
> for both codes.  The agreement improved slightly, but was still off
> overall.  Another thing is that CP2K tends to produce lower energies

So the problem is not in SCC! In this case you can just print out the
Hamiltonian and Overlap matrices with both codes and look for differences.
>From there it should be easy to identify the problem.


> than DFTB+.  I haven't quantified this at all, but that seemed to be
> what was happening when I eyeballed the results.
>> Could it be that your DFTB+ run is using a l-dependent
>> hardness by default? CP2K has only a single hardness
>> value per atom implemented.
>> Or is DFTB+ taking another hardness value from the
>> input file, CP2K is using the s-value.
> DFTB+ has an input flag where the code will only use the Hubbard U for
> the s-shell.  I assume this is the same thing as you are talking
> about, above.  I used that flag for the comparisons I'm talking about.
> Any other thoughts on other things I could look into?
> Thanks for the help,
> Nir
> >

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