[gmx-users] Question about scaling charges

[Arthur Roberts]

Hi, all,

     I was talking to a friend of mine that works in
an MD lab.  He said that the charges from QM/MM or
semiempirical need to be scaled to charges in the
force field.  For example, a heme will have specific
charges in the force field, while hemes that represent
different bound states may have a different charge
distribution, depending on which method is used to
calculate it.  Let us say that the force field charge
is +1 for simplicity and the charge is +2 by using
other methods.  The scaling would suggest that you
would reduce the charge of the heme by one half, so
that it would be compatible to the force field. 
However, this doesn't make sense to me, since partial
charges should always be related to an electron and
therefore, in principle, should never have to be
scaled.  I would appreciate anyone's input on it.

Best wishes,
Art