# Re: Negative Forces in SMD Simulations

From: Peter Freddolino (petefred_at_umich.edu)
Date: Tue May 26 2020 - 15:42:59 CDT

I would highly recommend reading the section in the namd manual on SMD,
which goes into a lot more detail. The most important point is that force
is a vector; in the particular case of the SMD pulling force, it will be
exerted along the SMD pulling direction; a positive value indicates a force
being applied along the pulling direction (because the motion up to that
point in the simulation has been less than it should be for the specified
pulling rate), and a negative value indicates that the force is applied
directly opposite the pulling direction (because the motion up to that
point in the simulation has been more than it should be for the specified
pulling rate). If there are other forces in the system that are pushing the
atom along the SMD reaction coordinate, you may well get a negative force,
as you effectively have to hold the atom back in order to reach the
specified pulling rate. Similarly, in the ball example, because gravity is
pulling the ball down, you would end up needing to exert a constant upward
force (after the first tiny interval as the ball accelerates to 1 cm/s) in
order to maintain the constant velocity, balancing the downward
gravitational force. This would, in the SMD convention, be reported as a
negative force, because it is exerted opposite the 'pulling' direction
(down -- 0,0,-1).
Best,
Peter

On Tue, May 26, 2020 at 4:24 PM Souvik Dey <sdey8_at_uic.edu> wrote:

> Hi Peter,
>
> The force in SMD convention according to this tutorial (
> http://www.ks.uiuc.edu/Training/Tutorials/namd/namd-tutorial-unix-html/node18.html)
> is the difference between the expected and the actual distance covered
> multiplied by the force constant. So, in this case, if you measure the
> position after 1 s, it should have moved 1 cm in the z direction. So, net
> force should be zero.
>
> Please correct me if I am wrong.
>
> Regards,
> Souvik
>

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