I'm going to perform a classical MD simulation on a graphene oxide nanosheet. Everything is ok when I run the simulation without considering coulomb interactions, but when I consider them, the volume diverges (constantly increases until infinity). What is going on here?
Every time the Coulomb potential is included in the calculation, it introduces infinities. When you consider the partition function of the hydrogen atom it diverges, if you consider the coulomb cross sections, it diverges and so on. It depends on the fact that the coulomb potential is very high also at high distance but also you can have problems at short distance. So you have to limit your potential in some ways. At short distance, you cannot have a point particle, but you should consider that these particles are spheres of the size of the De Broglie length. At long distance you can cut the potential at the Debye length or you can use the debye potential which avoid the divergencies. Attache find a paer treating similar problem
Every time the Coulomb potential is included in the calculation, it introduces infinities. When you consider the partition function of the hydrogen atom it diverges, if you consider the coulomb cross sections, it diverges and so on. It depends on the fact that the coulomb potential is very high also at high distance but also you can have problems at short distance. So you have to limit your potential in some ways. At short distance, you cannot have a point particle, but you should consider that these particles are spheres of the size of the De Broglie length. At long distance you can cut the potential at the Debye length or you can use the debye potential which avoid the divergencies. Attache find a paer treating similar problem
It is not quite clear from your question what precisely you are doing, but other possibilities beyond the ones pointed out by Gianpiero exist. First of all, for your particular case it doesn't matter that the partition function for the hydrogen atom doesn't exist or that the Coulomb cross-section diverges. Yes, the 1/r potential is nasty in many ways, but that need not be the source of your troubles. Also, your particles are likely neither of the size of the DeBroglie wavelength, nor of the thermal DeBroglie wavelength, which for Carbon at room tenperature is about 0.3 Angstrom, but rather have their usual van der Waals radius, which is about 1.7 Angstrom, and I'm sure that this is properly accounted for by some sort of short range repulsion in your model, say a Lennard-Jones hard core.
If you screen the Coulomb potential at large distances, you change the physics, and you better know exactly why you do that. Including Debye screening is appropriate if your Carbon sheet is embedded in an electrolyte, but if it is not, then screening requires a different justification, and you might not have one (other than desperation, and that's never a good one).
If I might guess, I suspect that in your problem the simulation box is not charge neutral. If so, then of course it would like to expand. But even then, this is often prevented from happening since you also have attractive interactions between atoms (chemical bonds) which usually hold the system together. Since you talk about a graphene sheet, however, I suspect that in the direction perpendicular to that sheet, call that the z-direction, there is no connection across the box, and so if you permit the box in z-direction to change length, it will. In any case, you should have a neutral system.
Incidentally, with long-ranged 1/r interactions you need to be careful how you handle periodic boundary conditions. Unfortunately, these forces are so long ranged that contributions from all other periodic replicas contribute to the force you care about, and you need to resum this. For three-dimensional periodic boundary conditions, this is typically done using Ewald summations. But then, you probably don't even want periodicity in the z-direction, because you care about a single sheet. If you periodically close your box only in two dimensions, x and y, standard Ewald is not the right way to proceed and needs to be corrected. Or you need to use Lekner sums.
Dear Sayyed:
I'm not very satisfayed with equations from quantum mechanics because the problem you adress here. So I'm currently reviewing a mechanical model for molecules. It seems interesting because it explains many things quantum theory cannot explain. If you see this model, probably you will be able to have better simulations of molecule behavior. Try searching "Miles Mathis" in web. The article "how elements are built" is particulary interesting.
I fully agree with Markos' answer. His comments are right on target,
Dear all
The problem solved using standard ewald summation instead of pppm for coulomb long range interactions. Thank you guys.
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