tclForces: relation between force and velocity

From: Westley A Sherman (
Date: Thu Feb 08 2007 - 12:55:19 CST

I'm trying to understand the force / velocity relations in tclForces
enough to add an external magnetic field (i.e. to apply Lorentz forces to
the charged atoms in the simulation). Eventually I'll probably have to
delve into the NAMD source code but it would help me get started to know
the answer to the following question.

Is it possible to use tclForces to bring an atom to a complete stop
(velocity of exactly 0) in a single time step and, if it is possible, what
is the best way to do it?

Although the tclForces interface has a "loadcoords" command, there does
not seem to be an equivalent "loadvelocities" command. A velocity can be
calculated using the coordinates from the current and previous time steps
but this may be less efficient (and even less accurate?).

Is the force necessary to bring an atom to a complete stop simply a
multiple of the distance between the atom's coordinates in the current
time step and its coordinates in the previous time step? If so, what is
the exact value for this multiple?


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