[DFTB-Plus-User] DFTB (Hbonding damping function + 3rd Order energy term)

[cmaupin at hec.utah.edu]

Dr. Bálint Aradi,
   I have successfully simulated water using the DFTB+ code with and  
without the hydrogen bonding damping term (DampXHExponent = 4.88) and  
can closely reproduce the oxygen-oxygen RDF found in J Phys Chem A.,  
2007, 111(26): 5685–5691. When I attempt to also use the 3rd order  
energy term (using DampXHExponent = 4.88, HubbardDerivs O=-0.14  
H=-0.05, these values were taken from J. Phys. Chem. A., 2007,  
111(42): 10861-10873) the simulation box forms voids.  The  
configuration that I start with is the final configuration from the  
previously described simulations (128 water for ~100 ps with a 1fs  
time step at ~300K).  It appears that the water density is too high  
(the waters are overly coordinated) and that causes the formation of  
voids or pocket in the simulation cell.  I tested the code on clusters  
and was able to reproduce the energies reported by J. Phys. Chem. A.,  
2007, 111(42): 10861-10873.  I am unsure if these results are real  
(i.e. no bug in code or input file) or if there is another problem.   
As I stated before the basic simulation parameters that I used for the  
original and hydrogen bonding damping simulations were used for the  
hydrogen bonding damping + 3rd order energy term simulations.  So, I  
think the input file is good.  I have checked to see if my 2x2x2  
MP-Scheme was big enough by calculating a 4x4x4 and 6x6x6 MP-Scheme  
and there was no difference between the output files (this is for the  
supercell calculations).  The SCCtolerance is set to 1.0 E-6 and these  
simulations do not use a thermostat (NVE).  Do you have any  
suggestions as to how I should proceed?  Also, has anyone ever run  
simulations utilizing both the hbonding damping term and the 3rd order  
energy term?  The above divide and conquer simulations (J Phys Chem  
A., 2007, 111(26): 5685–5691)only compared the original parameters and  
the Hbonding damping function improvement.
Any help would be greatly appreciated.

Best wishes

Mark

-- 
C. Mark Maupin, Ph.D.
Postdoctoral Researcher
Center for Biophysical Modeling and Simulations
U of U, Dept of Chemistry
315 S. 1400 E., RM. 2020
Salt Lake City, UT 84112-0850
Phone:  (801) 581-8606;  Fax:  (801) 581-4353
email : cmaupin at hec.utah.edu