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

[thassine at messi.uku.fi]

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.

Regards,

        Tommi