DFTB charge equilibration discrepancy

Nir nirgo... at gmail.com
Fri Jan 9 00:01:44 CET 2009

Hi All,

I have been benchmarking the DFTB implementation in CP2K against a
version of the DFTB code from Prof. Frauenheim's group (www.dftb.org)
and have run into some problems.  FYI, the DFTB code I used force
matches to DFTB+ to within 1%.  Specifically, for this problem we are
interested in systems containing C, N, O, and H.  For the benchmarking
tests I'm talking about, both codes were run using charge
equilibration (scc) and Ewald sums, without dispersion, and using
identical parameter files.  Similar very low SCF convergence criteria
were used for both codes.

For most systems I tested (e. g., CH4, H2O, CN, diamond), the two
codes compared very well and the forces and computed energies matched
to within ~1%.  However, the forces and energies were considerably off
for systems containing C-O, N-H, and N-O bonds (e. g., CO, NH3 and
NO2, respectively).  I hacked into both codes and saw that the
repulsive forces are virtually identical for both CP2K and the DFTB
code, but that for some reason they equilibrate to different charges
for the above systems.   Any help or insight into solving this problem
would be greatly appreciated.  I uploaded a tar file called
nir_dftb.tar of sample input and output files for a single NH3
molecule containing the following files (with hopefully self-
explanatory names):


In case it matters, I ran cp2k.popt on a single node on a Linux-x86-64
machine compiled with the intel fortran compiler.

Thanks for your help!


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