Cohesive energy and Solubility parameter

Hello LAMMPS users,

I am interested in the cohesive energy between the polymer chains and later use it to determine the Hildebrand solubility parameters as given below.

Solubility = sqrt ( E_coh / Volume)
E_coh = summation (E_iso) - E_tot; E_iso is the potential energy of each chain in the system and E_tot is the energy.

Here are list of things I did:

Method-1) Generate an equilibrated polymer box with 5 chains. Use the last frame to determine the potential energy (run 0), which would be E_tot
Later, I have created 5 independent datafiles, for each polymer chain, using exact polymer configuration and simulation box to determined energy of the polymer chain (E_iso). I have determined E_coh as mentioned above.

Of course, this procedure is not ideal when there are several chains and several frames to determined the solubility parameter.

Method-2) To determine the solubility parameter, on the fly, while the simulation is running.
In the same example, I have defined five groups and determined as shown below.

I think the main problem here is the kspace contribution, which you probably handling differently in the three methods. I suggest turning off kspace and redoing the intramolecular energy calculations. You should be able to recover the same values for each molecule using all three methods.

Hello Aidan,

Thanks for your response.

My pair style is “lj/charmm/coul/long/soft 2.0 0.5 10 9.0 12.0”.

I tried to remove kspace using “lj/cut/coul/cut/soft” (lj/charmm/coul/cut/soft is not available), the individual energies are quite high.

Also, turning off Kspace for a polymer chain for 40-50 Ang, how does it evaluate the potential energy if the cutoff is just 12 Ang. Unless I consider a huge box, with cutoff slightly less than half-of-the box length. Sorry, If I missed something here.