Dear LAMMPS developers and community,
I have a question regarding fix deform with remap v for fluids. Several notes in the documentation and in this forum state that the fluid velocity gradient has to follow the deforming box, one example from fix deform itself: “If you do not use a thermostat, then there is no driving force pushing the atoms to flow in a manner consistent with the deforming box. E.g. for a shearing system the box deformation velocity may vary from 0 at the bottom to 10 at the top of the box. But the stream velocity profile of the atoms may vary from -5 at the bottom to +5 at the top.”
If I understand it correctly, Lees-Edwards boundary conditions come with the velocity remap and a certain shift of position upon passing the boundary. The latter is taken into account by deforming the box, if I am not mistaken. The former, the velocity remap, happens in Domain::pbc() and its sections around if (deform_vremap..., where we add or subtract the velocity of the upper box edge.
Now what I do not understand is the question of where in this procedure it matters that the velocity goes from 0 to max instead of from -max/2 to max/2. Is it of consequence for the shift of position upon traversing the boundary? Of course it is relevant when we use compute temp/deform and related functions that rely on exactly that. But say if we shear a simple LJ fluid and thermostat it with compute temp/partial or compute temp/profile?
I hope this rather conceptual question is welcome in this forum and thank you in advance for your feedback.
Victor