I have some DFT data for the variation of binding energy with interlayer distance between two 2D layers. I would like to fit these values to an LJ potential to describe the interaction. I first divided the binding energy value by the number of atoms in one of the 2D layers to make it an intensive quantity and I obtained the epsilon and sigma for the LJ potential. However, when I implement these parameters in LAMMPS to describe the interactions between both layers the energy minimum is different from epsilon even though the minimization converged. I've noticed that as well in several publications such as:Article Mechanical Properties of MoS2/Graphene Heterostructures
The value of epsilon is around 3.95meV but the binding energy curve shows a minimum at 22meV. Any guidance regarding the difference between epsilon and the actual binding energy minimum in the simulation is appreciated.
Thank you
Dear Khaled Badawy ,
If I understood properly (if not, just ignore my entire answer!) you used two finite, ergo non-periodic, 2D layers with DFT to get the parameters you need for the LJ potential to be used in the periodic calculation. And that the final result you got is not the expected one. If all of that is correct, maybe the problem is that the initial non-finite layers were not sufficiently large to properly account for the interactions, even if you divided by the number of atoms to "compensate" the effect of size.
One possible solution is to repeat that first calculation with larger layers, and with all the results you may try to extrapolate the values for layers of infinite size, which should be more comparable to a "fully" periodic system.
Hope you find it helpful
Could temperature play a role here? Normally, force field parameters are derived for zero Kelvin and hence minima will differ when parameters are actually used at elevated temperatures.
Norbert Lümmen Thank you for the reply.
I use the LJ between the layers only and I increase the cutoff until the energy does not change. Also, I increased the vertical dimensions of the box so that the periodic image in z does not affect the calculation but I still do not get the value of the epsilon per atom. I get it an order of mag. higher
Norbert Lümmen Thank you for the very useful advice. Apparently the issue arises from the contribution of neighboring atoms. Is there is a way to take those into account while fitting the data from DFT ? If you have any references regarding this topic I would be grateful if you provide me with them.
Best Regards
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