[Chimera-users] substituting atoms

Elaine Meng meng at cgl.ucsf.edu
Tue Jan 9 10:24:09 PST 2018

Hi Simon,
I think you may be stretching the tools beyond their intended purposes: Chimera’s minimization, molecular dynamics interface, and automatic parametrization of nonstandard residues with Antechamber are meant to provide relatively simple access to such calculations by noncomputational scientists, given the large learning curves of dedicated simulation packages like GROMACS, AMBER, etc. These tools can be highly useful to clean up structures or suggest additional reasonable conformations. However, they are not meant for precise quantification of energies or for very long simulations, especially not in conjunction with nonstandard residues with parameters estimated by Antechamber.  The Gasteiger charges are very quick-and-dirty, so I would caution against overinterpreting simulations that employ them.  Further, Antechember is meant to cover most small organic molecules but not every possible molecule.

It seems that the modified UNK residues fall into two structural types (as per the “specify net charges” dialog), and that one (maybe just the 4th?) fails in parametrization by Antechamber, and then the Solvate or Add Ions tool cannot find some parameter needed for its calculation. These tools come from the Ambertools package even though they are included with Chimera and Chimera has interfaces to them.  I don’t know exactly what the problem is, nor do I have a solution, sorry.  
Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Jan 9, 2018, at 1:30 AM, simon chapman <rowanlodge19 at gmail.com> wrote:
> Hello Elaine, sorry to bother you again.
> Chimera has been a real treat so far, but I have hit a problem. I've struggled for a week now, and finally given up!
> I am incrementally substituting S for O on four guanines in a regular complex. 3 substitutions work fine in a simulation and producing very encouraging data (attachment 1) But 4 substitutions (attachment 2) triggers a load of error messages. The first is attachment 3. After solvation, attachment 4 appears and the solvation box is removed if neutralising ions are added. If ions not added, attachment 5 warning appears.  Not sure if attachment 6 is relevant.
> Hope you can help/advise?
> Best wishes   Simon

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