Question on "Simulating a free surface under vacuum"

Hi,

This was a long time ago, so I am not sure about the specifics in this case. However, I am pretty sure that your query is answered directly in the thread you shared with me:

The initial velocities of atoms at region surface1 and surface2 are not ZERO due to the velocity create command. Even though you set their force to zero, they can still move around. Set the velocities of your surface groups to zero can solve the issue.

You need only enforce a zero component that is normal to the free surface, aka :

velocity surface1 set NULL 0.0 NULL
velocity surface2 set NULL 0.0 NULL

however the actual boundary conditions you use will depend on what you want to do!!

Sometimes, the ideal case is a so-called ‘traction-free’ boundary, however I am not entirely certain how to do this off the top of my head.

Please post questions to the LAMMPs thread in the future.

Kind regards,
Nathaniel