Hello Junhui--

> 
>   I am doing ensemble refinement with PRE restraints using the fit.py script
> file. My protein is hexamer with flexible tails. I found that during the 
> cooling
> steps, the VDW energy was very large and the time step is very small. 
> Following
> is an example.
> *--- Dynamics ---- step=    300 ---- time=  0.021599 ---- delta_t=9.3514e-05 
> --*
> | E(kin)+E(poten)=     31384.160 E(kin)=       576.838 temperature=     
> 50.113 |
> |   E(poten)= 30807.3227460       grad=    34.6660233      ANGL=  
> 5162.0899430 |
> |       BOND=  1152.4255825       IMPR=  1012.4482395       PRE=  
> 2831.6677982 |
> |       RAMA=-21179.4077831        VDW= 41828.0989658
> 
> The starting structure was refined with no nonbonded violations and small VDW
> energies as below.
> *-- POWELL ------ step=    135 --- stepsize=   0.01000 --- energy evals=   44 
> -*
> |   E(poten)=  -929.6124511       grad=     1.8164828      ANGL=  
> 1041.7818070 |
> |       BOND=   226.5438108       IMPR=   137.7415772      RAMA= 
> -3098.5804725 |
> |        VDW=   762.9008263                                                   
>  |
> *------------------------------------------------------------------------------*
> 
> Is that possible that the VDW energy so high that the PRE energy can not be
> refined?
> 

I suspect the problem is with copies of the tag interacting with each
other. I strongly suggest that you instead work from this script in
the Xplor-NIH distribution:

  eginput/pre/refine/newRefine.py

and do write back if you continue to experience difficulties.

best regards--
Charles
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