[gmx-users] user-defined proper dihedrals and restraints

[K.A. Feenstra]

Chris O'Brien wrote:
> 
> Hello,
> 
> I have two questions.  First, is it possible to edit/create a user-defined
> function for proper dihedrals?  My molecule (polylactic acid) is chiral, so
> the dihedral rotation energy curve (phi vs theta) is not symmetric.  Thus,
> a Ryckaert-Bellemans expression can never fit it properly.  It would need
> phase shifts in the theta terms.  If this is possible, where in the program
> would I need to make these additions?  Are there any pitfalls I should be
> aware of?  If I were to make a piecewise (-180 to 0 and 0 to 180)
> expression, do I need to worry about discontinuity or non-differentiability
> at the junction?

I'd say you should be able to do about anything you like in the current
implementation, using either the R-B or simple sinoidal functions by
defining a number of functions to the same atoms. That way you can get
a sum of any number of (co)sine functions, phase shifted and/or with 
different multiplicities.


-- 
Groetjes,

Anton
 ________ ___________________________________________________________
|        | Anton Feenstra                                            |
| .      | Dept. of Pharmacochemistry - Vrije Universiteit Amsterdam |
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