[ghemical-devel] 1,4-scaling in ghemical tripos-5.2 forcefield

[Tim Vandermeersch]

On 3/13/07, thassine at messi.uku.fi <thassine at messi.uku.fi> wrote:
> Quoting Tim Vandermeersch <tim.vandermeersch at gmail.com>:
>
> > openbabel_benzene.log: same for openbabel
>
> This file was not right? Well it does not matter...
>
> > Comparing the log files:
> > - for VDW interaction 1, 5 and 9:   0.5 * E_ghemical = E_openbabel
> > - these interactions are the 1,4 interactions for the carbon atoms in the
> > ring
> > - openbabel scales them, libghemical does not.
> >
> > Is this a bug, or should these interactions not be scaled?
>
> I looked what happens in benzene and butane calculations with
> libghemical/tripos5.2 and everything seemed to be OK. The 1,4 terms are
> correctly detected and scaled so that the van der waals energy scaling factor
> is 0.5 and electrostatic energy scaling factor is 0.5 (this does not affect the
> benzene case).
>
> If you wish to study this further, you can look at tab_mm_tripos52.cpp around
> line 620 : this is where the 1,4 scaling is done for the parameters that later
> go into computations.
>
> I also added eng1_mm_tripos52.cpp line 871 in svn "trunk" version which (if
> uncommented) makes output about all detected 1,4-nonbonded-terms.
>
> Could it be that openbabel scales the interaction twice, because in benzene
> there are two separate paths that makes these atoms 1,4-related? As you can see
> from the above source files this "double scaling" cannot happen in libghemical
> because we can be sure that the scaling is applied only once.

Yes, this was the problem. Thank you very much. And sorry for doubting
ghemical :)

Tim