I'm doing a simulation on an NVT ensemble with a single polymer chain centered in the simulation box whose half-length is considerable larger than the maximum length of the polymer, so the chain doesn't interact with periodic images. I'm also setting the initial velocity of each monomer at 0, thus the initial momentum is zero too, and I'm not including any external force. From such initial setting, one expects that the net velocity of the system remains zero through the entire simulation, and the center of mass of the molecule doesn't move at all, only that the molecule folds/unfolds and rotates. However, when I run my simulation with Espresso, that doesn't happen. At the beginning I thought that it was because of the cut-off radius, so I set r_cut to a distance larger than the maximum length of the polymer, but the result was the same. I'm not very familiar with Langevin thermostats, so I don't know if the problem is my initial setting or if it is a pathological issue of the algorithm. I'd appreciate if someone can help me with this problem.