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henniggroup / VASPsol

Solvation model for the plane wave DFT code VASP.
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What are the units for surface tension "TAU"? #28

Closed ehermes closed 9 years ago

ehermes commented 9 years ago

I would like to use VASPsol to simulate solvation in methanol. It appears that VASPsol will default to water as a solvent, and will default to a surface tension of 5.25E-4. However, I am not sure what the units are on this. I have checked eV/A^2, hartree/bohr^2, and every combination in between (including Rydbergs), and none of these seem to line up with the default value in VASPsol if the surface tension of water is taken to be 72.8 mN/m. Therefore, what are the units for "TAU"? What should I set as the value of TAU if I want to simulate methanol, which has a surface tension of about 22.5 mN/m?

matk86 commented 9 years ago

the units for tau is ev/ang^2. Tau in the model is a fit parmater, not the macroscopic surface tension. It is basically a correction term that accounts for non-electrostatic effects. As long as the electrostatic effects dominate the solute-solvent interactions, the default value for tau is fine for a range of solvents.