# Which pressure coupling method to choose?

Hello,
After through study of manual and many research paper I prepared my simulation file to calculate the interfacial properties of oil-water system in presence of a surfactant and alocohals.

The following confusion arises when I see literature and manual for pressure coupling option.
The algorithm in manual says:
" When a periodic system consists of more than one phase, separated by surfaces which are parallel to the xy-plane, the surface tension and the z-component of the pressure can be coupled to a pressure bath. Presently, this only works with the Berendsen pressure coupling algorithm in GROMACS. The average surface tension Îł(t) can be calculated from the difference between the normal and the lateral pressure"

1. This means that pcoupl should be Brendsen and `pcoupltype` should be surface tension? Right?

2. However, What I found that many researchers have used `Parrinello-Rahman` for instead of Brendsen and reported results in paper. This shouldnâ€™t be right as per manual. I did it for my system using both and grompp didnâ€™t complained any time. The results were pretty close though.

3. Not only that, i found people have not used surface tension coupling and used semi-isotropic and still were able to compute surface tension.

I searched old forums too. Nobody had a clear answers there. Few people had suggested that it might have been implemented but may not have been updated in manuals.

1. Another point I would ask is about ref p and value
As manual says for surface tension coupling :
Two ref p value are needed
"The first -ref-p value is the reference surface tension times the number of surfaces `bar nm` , the second value is the reference z-pressure `bar`"
many time a constant pressure of 1 bar and zero compressibility is used in xy plane ( first ref-p value).
what reference surface tension should we be using here?

Reading the MDP option for `pcoupltype`, the `surface-tension` setting changes the box size to set a reference surface tension value for the interface. That (times the number of interfaces) is the value you supply as `ref-p`.

If you want to measure the surface tension at a specific pressure, use `semiisotropic` instead. You can then calculate the resulting surface tension using the relation from the manual or with `gmx energy`.

Since when we add surfactant, IFT changes drastically. Can we give any arbitrary value since we donâ€™t know the actual surface tension??

Sorry, how do you mean? An arbitrary value for `ref-p`? Likely not. If you donâ€™t know the surface tension you need to measure it at your desired pressure, using `isotropic`, `semiisotropic`, etc.

Per the coupling issue, read the original paper by David van der Spoel and his team. Also, read this one too

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I would use semiisotropic coupling with incrompressible settings along x/y, so the surface tension does not affect the box scaling.

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Thanks for the literature suggestion. I have read the second one earlier before preparing the mdp file.
@hess
I would use the same setting as I found in numerous research papers. I only wanted to know weather it is neccessary to use surface tension coupling with brendson option as mentioned in manual or not. The answer for which I couldnâ€™t find anywhere

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