Colvars "tilt" component force constant units for WHAM

From: Robert Elder (
Date: Tue Jul 12 2011 - 16:36:55 CDT

Hi all,

I'm a little unclear about the units of the force constant (K) for the new
'tilt' collective variable. The units of the output for this colvar appear
to be cos(angle in radians), such that I can extract the angle in degrees
using acos(colvars output) * (180/pi).

I have been using the raw output from the colvars module (i.e. cos(radians))
and the force constant in my colvars configuration file (no clue what the
units are) for WHAM. However, it appears to me that using this force
constant may be incorrect, based on a) the expected results of the
simulations and b) some testing with lower force constants, which show
behavior more closely resembling what I expect. So this made me wonder if I
need to convert the K from colvars.conf to some other set of units where the
value of K would be lower. Of course, I don't want to pick and choose random
numbers until I get desirable results!

So, my question (maybe Jerome or Giacomo can comment): do I need to convert
the tilt force constant to some other set of units for WHAM? Or should I
just continue to use the value I have in my colvars.conf file?

I can provide more details if necessary.


Robert Elder
University of Colorado at Boulder
Chemical and Biological Engineering

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