Surface tension of water model

I want to calculate the surface tension of a pure water model. For that I created a system and created an interface by extending the box length in the z-direction. After that I run an nvt simulation Now I am getting the value double the surface tension of the water model, Can anyone please help me.

You have two interfaces, so you divide the resulting surface tension by two. If you’re getting double what you should, that means your result, after division, is correct.