[gmx-users] Strange behavior of water droplet on a solid surface with Verlet cutoff scheme in GMX 5.0.4

[Jan Jirsák]

Dear community,

I simulate a droplet made of rigid SPC waters (ca 10000 molecules)
lying on a solid surface placed at z=0 represented by LJ atoms with
fixed coordinates. Using cutoff-sheme = Verlet with quite long cutoff
(3.0nm) produces a strange spurious force, which seems to pull the
droplet towards the origin (or maybe in the -1,-1,0 direction). It is
not caused by particular configuration, because even when I rotate the
setup by 180 deg., the strange force still pulls the droplet in the
same direction - to the origin, i.e. in the opposite direction
relative to the configuration.

When I decrease the cutoff to, say, 1.9 nm or when I use a group
scheme instead, the artifact disappears.

Does anybody have a clue what could be happening? Does anybody observe
anything similar?

With kind regards,
Jan