Hi,
define can change the value of internal coordinates if the Pulay internal coordinates are used and not the redundant ones. If you manage to get a complete set of internal coordinates (e.g. by using
iaut and then
imet to check the linear dependency), it is possible to change the value of each internal coordinate with
iman.
For larger molecules or such with a more complicated structure (annealed rings, etc.) where the automatic generation of Pulay internal coordinates with
iaut fails, another approach can be used:
- read in your structure in define as usual
- define the internal coordinate you want to change as fixed (idef and then e.g. f tors 1 2 3 4 , bend 3 1 5, ...)
- generate internal redundant coordinates (not really necessary, but it makes sure that all keywords are written to the control file by default)
- continue with the input generation as usual
- after define has finished, open the coord file and search for the
$intdef
# definitions of internal coordinates
1 f 1.0000000 bend 4 2 1 val=92.34832
section and change the value to whatever you like (too large changes can result in problems with convergence, see below). - re-run define, enter the geometry menu and just redo ired
The internal redundant coordinate generation step automatically reads in all fixed internal coordinates and checks if their values differ from the cartesian structure. If that is the case, it will iteratively try to move the cartesians to the new values of the internal coordinates (cartesian -> internal is trivial, but internal -> cartesian is not uniquely defined). Check the output if the structure has converged, if not retry the procedure with smaller changes.
This is also described in the tutorial.
Uwe