[ghemical-devel] Bond Length Constraints

[Donald Ephraim Curtis]

I was wondering in what way i could implement bond length constraints
that would satisfy the ghemical community (code and interface).  I
realize this is a much more complex issue as it effects other areas of
calculating certain properties, however, I believe we can implement the
interface changes in much the same we way did the frozen atom support.
However, my main concern is the libghemical changes which I believe in
this case require changes to the engines.  The way we handled this
previously was when we calculated the bondlength we raised that constant
and introduced our own properties about the bond that would set the
specific length.

Basically, we created a constraint list that included the two atoms
which were constrained.  We did this because we wanted both bonded and
non-bonded atoms to be able to constrained.  This alleviated the problem
of having to bother with angles and torsions (is that right in chemistry
terms?).  Also in that previous case we also made it so non-bonded
constrained atoms were treated as though they WERE bonded.  It was just
a work around since the non-bonded code got in the way.

In the end we simply had a check in the ComputeBT1 function that would
set the optimum distance to the value set by the user.  Since angles and
torsions have a lighter effect on the bond lengths we didn't even have
to bother with the constant.

Thanks in advance,

Donald Ephraim Curtis