Colvars: Using "Alpha" Component

From: Alexander Predeus (
Date: Thu Sep 23 2010 - 19:26:31 CDT

Hello All,

I'm interested in using a collective variable based on "alpha"
component of colvar module, and I am running into some kind of
persistent error. The message is as follows:

> Error: one or more of the components of colvar "a1" is unable to calculate system forces.

The error persists when I'm using both 2.7b3 and 2.7b4 versions, and
two very different systems (~ 500k atoms and ~ 20k atoms). The colvar
config file is listed next:

colvarsTrajFrequency 10

colvar {
        name a1
        width 1.0
        outputVelocity on
        outputAppliedForce on
        outputSystemForce on
        alpha {
                residueRange 1068-1075
                psfSegID PROA

harmonic {
        colvars a1
        forceConstant 50
        centers 0.5
        #targets 1.0
        #targetsNumSteps 2000

I've tried playing with other parameters that one can change for alpha
component and nothing seems to help me get rid of this error. I've
also compiled NAMD with -DCOLVARS_DEBUG option, which does make it a
lot more talkative, but doesn't help in this particular case; the
error message remains the same.

Any suggestions would be appreciated.

-- Alex Predeus
Michigan State University

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