SMD-drifting of center of mass

From: Ekta Khurana (
Date: Sun May 01 2005 - 16:47:19 CDT

  Hello all,
       I am using SMD to pull the center of mass of a group of atoms in z
direction with a constant velocity. In the log file the force in
x,y direction is zero as expected and a finite value in the z
direction. However in the trajectory if I see coordinates of the
pulled center of mass, the x,y components are also changing besides
the z component. I tried a more stiff spring (higher force
constant) than what I was using but the problem remains. I tried
force constant value of 250 (in units used by NAMD that is
kcal/mol/angstrom^2). Could anyone suggest why this is happening?


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