Dear fellow Lammpers,
I have had some frustrating moments with the beloved respa integrator when doing npt simulations of a mixed phase system. Basically the box keeps growing until it has reached cosmic proportions … which is of course surprising and a little unphysical. In contrast when I use the humble verlet integrator, everything is stable, my system converges to the expected equilibrium density. As respa is very beautiful and very fast, this is very unfortunate. By the way for the system we are looking at, NVT/NVE with respa is fine, and reproduces the essentially the same results as verlet applied for the same ensemble. Any ideas of what might be wrong ?
The respa problem does not appear if use respa to simulate water at stp
(as you would expect).
The system consists of charged nafion polymers (which are pretty stiff) , corresponding hydronium molecules (with opposite charges to the polymers) and water. The system is overall charge neutral.