Dear all, I would like to create layer system composed of 2 surfaces where the first surface A is fixed while the second surface B should be movable in Z direction as the rigid body (i.e. in the direction perpendicular to the both surfaces) to find during the simulation equilibrium spacing between them. Unfortunately I am not able to achieve what I need - i.e. to ensure that in case of surface B it will be movable in the Z direction as the rigid body (without any deformation = all the relative distances of atoms in the B should be fixed while the whole B can move in Z direction). I tried to apply constraint  "Fix fractional position" on surface B which allowed it to move in Z direction in my layered system but in the same time it deformed i.e. it. did not behave as the rigid body. I would be really grateful for any useful information. Thank you very much in advance !  Marek Maly