Gradient tolerance nan for A three-atom simulation

From: accomp lin (accomplin_at_yahoo.com.cn)
Date: Mon Jan 05 2009 - 23:09:41 CST

Dear NAMD developer:
 
I've modifed a namd source code with New bond energy potentials for my Coarse Grained models. After I recompiled it successfully on linux, I constructed a three-atom model to test my new NAMD. Only bond and angle are included in the psf file, the three atoms are aligned straight, each of them have a mass of 400 amu. But when I run the program, the simulation held still and had the following error.
 
INITIAL STEP: 1e-06
GRADIENT TOLERANCE: nan
BRACKET: 0 0 nan nan nan
 
After several try and visiting the Mailing list I found I was the Angle term tha caused the problem. When I turned it off or set the Ktheta zore the problem dissappeared. I already set a large enough boundry box the contain all the atoms within it. I know it might be a little inappropriate to use NAMD do to such a simple simulation, for it can cause various problems. But I really want to know how does it work and how to solve these problems. So please help, thank you.

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