Gmx energy for solubility parameter

User discussions
1

GROMACS version: 2023
GROMACS modification: No

Hi all,

I am trying to calculate the hildebrand solubility parameter of pentane with GROMACS. I need the cohesive energy density which should be:

E_tot (bulk) - E_tot (single) = 31515 kJ/mol - 48.9 kJ/mol

but the resulting solubility parameter is over an order of magnitude lower than I expect (0.5 MPa^(1/2) vs 14.5 MPa). The density of my system is very close to experimental so the molar volume is okay and it must be the energy. I am using gmx energy to get each energy (pentane in bulk and pentane in vacuum), selecting the ‘potential’ option. Does anyone know if this is the correct approach or should I be looking into my system more?

Thanks.

2

I’ve never calculated the Hildebrand solubility parameter, so take my advice with a grain of salt, e.g., I’m not sure if the potential energy is what you should use in this case.

Anyhow, since you are only including the energy from one molecule in vacuum, you should do the same in bulk. How many molecules are there in your bulk system? Do your results get better if E_tot(bulk) = 31515 kJ/mol / [N pentane molecules]?