|From:||Axel Kohlmeyer <akohlmey@...24...>|
|Date:||Sat, 26 Aug 2017 16:06:37 -0400|
Frist of all, thanks for your benefit advices about testing the fix efield in lammps/reax/c. Following our previous discussions about fix efield in lammps/reax/c I setup some very simple systems as you said. The systems are as below:
1. Only one Na+1 atom is placed in the system and the fix efield is investigated when we have a L.J. potential.
2. Completely separated Na and O atoms with partial charges equal to that calculated by reax/c potential are placed in a system and this system is subjected for comparison the fix efield along with L.J. potential and reax/c potential in the Lammps software.
3. Test the second experiment (in completely similar condition) in the ReaxFF Fortran code.
In all simulations I have used 0.01 volt per angstrom external electric field only in x direction and 0.01 K temperature. The input and output files are attached to the email. I calculate the force(s) on the atom(s) due to the external electric field with hand and by using the displacements from the simulation results I calculate the electric field energy imposed to the system then I compare my results with the results gained from the above simulations and the final results are as bottom:
1. The fix efield in the Lammps software along with both classical pair potentials (e.g. L.J.) and reactive potential (ReaxFF) act in the same manner absolutely equal to the thing mentioned in the Lammps manual. I think the external electric field is a classical homogenous electric field in the Lammps software.
This makes no sense at all. Your first style is pure LJ (lj/cut) which has no charge-dependent terms therefore any efield has no effect on pure LJ. On the other hand, ReaxFF neutralizes the entire system then finds the equilibrium charges. If you only have Na atoms in the system, then all of the Na atoms would have zero charges – which means any efield has no effect at all.
2. In the Lammps software, in comparison with the classical potentials, when we use ReaxFF potential, we have smaller effects of the fix efield since the qeq/reax/c charge equilibration scheme produce partial charges on atoms during the simulations and this charges are very smaller than the initial charges at the beginning of the simulations but in the classical simulations we have constant charges (equal to the initial values) during the simulations. In a reax/c example, I set +1 and -1 charges on atoms but after the first step the new charges were approximately +0.3 and -0.3 respectively.
These results are expected and are reasonable.
3. When I use external efield in the ReaxFF Fortran code, It has some unpredictable results that are vague for me and also the results due to external efield are apparently different from the results gained from fix efield in the Lammps software in similar conditions. For example, the ratio of resulted delta_Epot in the ReaxFF to resulted delta_Epot in the Lammps package is about 120 for 100 steps. I will contact to ReaxFF developers about this subject.
Do you mean you are using the stand-alone ReaxFF code from Prof. van Duin? Make sure your efield has the same units.
Thank you for your beneficial reply to my questions. If you have any suggestion/correction or criticism on the above experiments and my descriptions, it's my pleasure.
Thanks in advance
Mohammad Ebrahim izadi,
Department of Chemistry,
Islamic Republic of Iran,
Phone : +98 – 21 – 61113358
Fax : +98 – 21 – 66409348
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