[gmx-users] interaction parameter-polymer solution system

Justin A. Lemkul jalemkul at vt.edu
Mon May 17 15:33:42 CEST 2010

Moeed wrote:
> Dear experts,
> I am interested in calculation of Flory interaction parameter for 
> ternary system of polyethylene/n-Hexane/ethylene.
> Flory-Huggins lattice theory for polymer solvent systems defines 
> interaction parameter (binary) in terms of interaction energies between 
> lattice sites.
> binary interaction parameter  =z*delta (e)/RT
> z=coordination NO. which is the number of neighbors in a lattice site
> delta e=e12 -(e11+e22)/2       : net change in interaction energies upon 
> mixing (polymer: polyethylene in solvent :hexane)
> e12 [kcal/mol] is the interaction energy between polymer and solvent (of 
> course after mixing) 
> e11, e22 interaction energy between identical molecules before mixing 
> (polymer or solvent)
> Since we are talking about energies between lattice sites my 
> interpretation is that only non bonded interactions should be considered 
> among all energy terms.(vdw and electrostatics).
> 1- However, I dont know if I need to include electrostatics terms since 
> the system is apolar (only hydrocarbons). Also, do LJ energy figures (or 
> coulomb) given by g_energy refer to e11,e22.. interaction energies? 

You are still using OPLS-AA, are you not?  If so, then your atoms have partial 
charges on them and therefore there are electrostatic interactions between your 
molecules.  You can't ignore this term.

By default, you will not get these interaction energies decomposed as you would 
like.  You can, however, set "energygrps" in the .mdp file to separate out these 
contributions.  If using PME, however, there will be a long-range term that 
simply cannot be decomposed, so choose your long-range electrostatics method 

> 2- To calculate these interaction energies, for instance for identical 
> molecules , Hexane,  how many molecules i need to put in a simulation box.

Enough to get convergence :)

> 3- Do I need to consider "free energy of solvation" for this type of 
> calculation?

Sorry, can't comment on this.  Maybe someone else can.  From the above 
equations, there don't seem to be any free energies involved at all, but I've 
got very little experience in polymers.


> Many thanks,
> Moeed


Justin A. Lemkul
Ph.D. Candidate
ICTAS Doctoral Scholar
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080


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