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Re^3: Problem of freezing a few torsion angles.

Alex Granovsky
gran@classic.chem.msu.su


Hi,

>I try to freeze all the torsion angles in the molecule and one error go after another. If I don't use the DLC all torsion is frozen by IFREEZ, but without DLC the cycles are broken, etc. unpleasant. When the DLC = 1 IFREEZ immediately stops working. I must use IFZMAT, but it only works when AUTO=1

The DLC=.t. (or 1) with AUTO=.f. (or 0) is the very specific
combination. It assumes that DLCs will be specified manually;
and indeed, almost all DLC-specific options will not have any
effect for these manually created DLCs. Thus, this is by design.

> and works in a strange way.
>In calculating the gradient of the frozen coordinates are at the top of the list, which consists of 30 coordinates for benzene (NZVAR ignored?) (if torsion not freeze, then this list no torsion)!

DLCs uses linear combinations of primitive coordinates,
and the number of primitive coordinates is generally greater
than the number of independent internal coordinates.  

As to torsions, Firefly replaces some badly-defined torsions
by the Out Of Plane (OPL) bends and this is why your input will not
necessarily generate torsions. However, if one constrains a
primitive coordinate, it must be in the primitive set, and will
be forcibly added if it was not initially generated; and will be listed first.

>If the molecule is initially planar occurs
> error: "ERROR: UNABLE TO PROJECT DLC."

Two comments here. First, the set of primitives must be
(over) complete (and not too ill-defined) after removing all
the constrained primitives off the set. This is not always true
for the automatically generated coordinates, so you may need to
add some extra coordinates. Second, this message may indicate
that some of your torsions are singular or nearly singular
(e.g., very close to 180 deg.). In this case, it is better to
use OPLs and constrain them as they are generally not singular
when torsions become singular.

>If it is not planar like this:
>"THE ROOT MEAN SQUARE ERROR IN ITERATION 1 IS 0.00003940
>THE ROOT MEAN SQUARE ERROR IN ITERATION 2 IS 0.00000000
>
>
>CONSTRAINT TRANSFORMATION IS NOT COMPLETE.
>ONE MORE ITERATION IS GOING TO BE PERFORMED. "

You may need to relax CNSTOL of $statpt group (the default of 1d-8
can be too tight sometimes).

Regards,
Alex Granovsky


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