I am conducting a simulation of a three-layer system consisting of surfactant, water, and oil using GROMACS. As part of the verification of the results, I am outputting the pressure in GROMACS to calculate the interfacial tension, but I am getting negative values. Is this normal?
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2 Answers
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In most simulations, negative pressure indicates the system wants to expand. In NVT(constant volume and temperature) ensemble it is common to get negative pressure values. However, even with NPT (constant pressure and temperature) ensemble, there are constant adjustments to the simulation box size to maintain the target pressure. Fluctuations around the target pressure are normal, and occasional negative values are expected. If the negative pressure seems to lessen(larger negative value) over time, it could be due to insufficient equilibration at the target pressure. Larger negative values could indicate problems with your other parameters as well.
In your case, negative pressure could arises from the outward pressure being exerted by the surfactant molecules, attracted to both water and oil. Additionally, water's inability to form bonds with oil creates a repulsive force at the interface, further contributing to the system's tendency to expand.
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Sure, you're right, a negative pressure is weird. The pressure related to surface tension, however, is just a relative pressure. See this photo of the page 892 of the Tester and Modell "Thermodynamics and its Applications":
To derive the equations of surface tension we must begin with an interface between two systems that may present different pressures. They call it $P^{\alpha}$ and $P^{\beta}$. From this pressures is derived a possible deformation of the interface:
The important point here is that we always refer to the pressures as relative when we talk about surface tension, say, the real absolute pressure in the medium minus a reference, atmospheric presssure.
The pressures you are getting are the absolute pressure minus the atmospheric pressure, that's what it can be negative.
On GROMACS it is a point of attention, because, as it is said in the tutorial, the software suppose that you are working under 1 bar. If your rel pressure is far from it you must adapt:
