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Alex Granovsky
gran@classic.chem.msu.su

actually, the problem is related to the algorithm used to create
hydrogen bonds to add them to a set of primitive coordinates.
Sometimes, code can create excessive number of H-bonds, especially
for molecules containing Sulfur atom(s). These extra H-bonds
typically negatively impact the number of required geometry
optimization steps. The scoring function used to assign H-bonds
has been recently changed to be much more more realistic so the
problem does not longer exist with latest source code. However,
with Firefly 7.1.G, the best way to speedup calculations of this
class of compounds is to disable automatic generation of H-bonds:

 $zmat hbond=0.0 $end

In addition, optimization is faster with default optimizer (gdiis).

Hope this helps.

Alex Granovsky


On Sat Feb 26 '11 10:01am, Khimich wrote
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>Thanks for the reply.
>But all bond lengths in the ring ~1.4 A and the geometry is close to optimal. But 55 steps of optimization has led to a decrease in energy by only ~0.4 kcal / mol.
>
>
>On Sat Feb 26 '11 0:06am, Amir Nasser Shamkhali wrote
>-----------------------------------------------------
>>Your input structure is too far from an actual structure. For instance, the C-C bond length is 2.65 angstrom in the ring.