ABF - restricting reaction coordinate

From: Subramanian Vaitheeswaran (vaithee_at_umd.edu)
Date: Fri Dec 07 2007 - 08:55:48 CST

Dear Chris and Jerome,

Following both your suggestions a couple of days ago, I split the range of my reaction coordinate (a dihedral angle) into four windows. I specify the following in my .conf file:
********************************
abf coordinate dihedral
abf abf1 {8 6 12 16}
abf dxi 1.0
abf xiMin 0.5
abf xiMax 45.5
abf writeXiFreq 50000
********************************
So, I expect that xi will be in the range [0.5, 45.5] degrees for *every* step.

But xi is almost never in this range - here is a small fragment grepped from the log file:
********************************
TCL: ABF> Xi at timestep 4300000 : -119.747074603
TCL: ABF> Xi at timestep 4350000 : -44.6413954893
TCL: ABF> Xi at timestep 4400000 : -55.1637937915
TCL: ABF> Xi at timestep 4450000 : -108.865140314
TCL: ABF> Xi at timestep 4500000 : -155.734544341
TCL: ABF> Xi at timestep 4550000 : -57.9804339106
TCL: ABF> Xi at timestep 4600000 : -160.710021078
********************************
Does this imply that the default value for forceConst of 10 kcal/mol/^2 is too small for this system? Do you see any problems with increasing it to an arbitrarily large value?

Thanks for all your help,
Vaithee

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