Hello all,
I am new to MD and GROMACS. I was faced with the task of calculating the surface tension. As far as I know, the calculation parameters in *.mdp file, such as rvdw, rcoulomb, etc. depend on the specific force field and are presented in the original articles for that force field.
However, the literature states that the surface tension depends on the cutoff of the Lennard-Jones potential, with the most accurate surface tension results obtained when the cut-off value is greater than 6 * sigma (for example, in the article Chem. Soc. Rev., 2016, 45, 1387-1409).
Therefore, I have a question: is it correct to use a larger cut-off (compared to that from original articles for particular force field) to calculate the surface tension? For example, 3.0 nm instead of 1.4 nm for GROMOS force field (or instead of 1.5 nm for OPLS-UA ff)?
I would appreciate any suggestions and thank you very much in advance
Forcefields are parameterized for specific rvdw. For example, the rvdw may have been determined to match simulation and experimental free energy value of water, density of water, or match with QM data. Increasing or decreasing (best to my knowledge) will only make the mode worse (unless detailed verification/validation is performed)
The same nonbonded cutoff is used for both electrostatic and van der Waals interactions for convenience: one common nonbonded list. But the distance dependence is different for the two types of nonbonded interactions: 1/r**2 for electrostatic interactions versus 1/r**12 for the repulsive and 1/r**6 for the attractive Lennard-Jones terms. Changing the cutoff from 14 to 30 Angstroms may have sizeable effect on electrostatic interactions (1/196 to 1/900) but unlikely on van der Waals interactions (1/56693912375296 ~ 0 to 1/531441000000000000 ~ 0 for the repulsive term; 1/7529536 ~ 0 to 1/729000000 ~ 0) which do not reach that far.
Forcefield parameters are derived to calculate a specific set of properties by minimizing the errors. The surface tension of water is 72 mN/m at room temperature. Lets say the forcefield1 predicts the value to be 72.5 mN/m, while forcefield2 predicts it to be 71.5 mN/m. Without the loss of generality assume increasing cutoff increases the surface tension. So for forcefield1 the prediction is worsened, while for forcefield2 the prediction is more accurate. You have to validate and verify if your forcefield is forcefield1 or forcefield2. If you think changing the cutoff will not change the surface tension prediction, then increasing the cutoff only increases computational load and is not ideal.
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