How to obtain the interfacial tension between two phases in discover ?

Hi,every one

I want to simulate the interface properties or interfacial phenomena in gas-fluid or fluid-fluid,and I know the formula of interfacial tension,this gives:

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In discover,if  use the  NVT ensemble ,the three pressures are not been added in the simulation; if the NPT ensemble is used,the volume of the box will

change or the Lz will be not a fixed value. So,it seems that there is not a way to obtain the interfacial tension,do you have some good suggestion about

this question? or how do you calculate the interfacial tension in MS,even in discover? I wish your answer.

Thanks

  qinjian