Dear Anand,

Although your question is good but I am still curious why do you need to simulate just water molecules?

Shaban Ahmad I was asked by my principal investigator to simulate the water molecules using TIP3P and TIP4P/2005 model.

In a cavity within what?

Martin Klvana I have to simulate a water droplet in a spherical cavity of 20 nm. The spherical cavity should have a spherical potential instead of periodic boundary conditions.

You should have some complex to simulate in water using TIP3 or other principles.

Ashutosh Anand, define and apply solvent boundary restraint; i don't know about NAMD, but it can be done with Q (https://github.com/qusers/Q6): the radial restraining potential has a half-harmonic term that pushes solvent molecules back into the sphere and a Morse-like term that pulls molecules from inside out towards the boundary.