RESTRAIN_HEAVIES_TO_ZERO and RESP

[Matt McGrath]

Hi guys.  I've been playing around with the RESP charges in a QM/MM
system, and some of them that I'm calculating are quite unphysical
(+17.0, for example), despite that the DDAP charges are pretty
normal.  They are only on the hydrogen atoms, so increasing the
RESTRAIN_HEAVIES_STRENGTH doesn't help (eventually the heavy atom
charges go down to zero while the unphysical hydrogen atom charges are
basically unchanged).

I've looked in qs_resp.F, and it seems quite easy to change it so that
all charges are restrained to zero (commenting out one IF statement).
However, this is not offered as an option in the input file, which
makes me wonder if there is a good reason for only restraining the
heavy atoms to zero.  Is there theoretically a problem to restrain all
the atoms to zero, instead of just the heavy atoms?

The other option is to restrain the problematic atoms to their DDAP
charges, but then one can ask why we don't just use the whole set of
DDAP charges.  I've considered that, but there seems to be very little
use of the DDAP charges in the literature.  I'm currently looking for
a good small molecule test set that I can compare the DDAP charges to,
in order to convince ourselves that the DDAP charges work well for
small drug molecules.  I guess restraining the problematic atoms to
their DDAP values seems like a reasonable action, since then the ESP
should be well-reproduced but all the charges will be reasonable.

Thanks!