Dear GROMACS user.
I am simulating a system consisting of Decane/water/surfactant. I would like to pull one surfactant molecule toward the water phase while the other surfactants remain intact, to calculate delta G using umbrella sampling (As I depicted in the attached image). I read the GROMACS tutorial for umbrella sampling. However, I don’t know how to pull just one surfactant. In the tutorial, the whole system was pulled while one part was constrained. How can I do that?
Also, Can I apply force on just one atom on the head of surfactant, for example?