CCL:G: Constraints?

 Sent to CCL by: =?ISO-8859-1?Q?=D6d=F6n?= Farkas [farkas]_[chem.elte.hu]
 Hi David,
 It is not really possible to say what to do in general in a case like
 yours. Many kind of problems may occur, like the bonding of the stacked
 molecules. To try to solve this choose two proper atoms (A1 and A2) in
 the two molecules and add
 A1 A2 B
 to get proper bonding between the stacked molecules. You might freeze
 the bond angles defined for this bond then.
 If zou have more choices to connect stacked molecules you might add all
 of them. Of course, this will just help the description of the motions
 of atoms with redundant internals and should not affect the result.
 If you have further questions contact me directly.
 Best wishes,
 Ödön
 On Mon, 2008-11-24 at 08:28 -0500, David Close closed[*]etsu.edu wrote:
 > Sent to CCL by: "David  Close" [closed(~)etsu.edu]
 >   I'm having a problem imposing constraints on a series of stacked
 molecules.  I'm trying to look at the influence of neighbors in a problem
 involving a crystal structure.  When I get up to three molecules in a stack the
 middle molecule tends to move away from the stack.  I need to impose
 constraints, but I keep getting a message "Curvilinear step not converged.
 Error in constraints".  Thuis is in GAussian.  On another system the
 program seems to fuss about symmetry?
 >   My questions is, what calculations are being performed to decided whether
 or not a constraint is suitable?  If one knows what calculations were being
 perfomred, is it possible to add suitable constraints? It seems as if a few bond
 constraints work, but when I add constrained angles, the curvilinear message
 appears after 100 "tries".>
 >
 --
 Ödön Farkas
 Associate professor
 Deparment of Organic Chemistry and
 Laboratory of Chemical Informatics,
 Institute of Chemistry,
 Eötvös Loránd University, Budapest
 Address: 1/A Pázmány Péter sétány,
 H-1117 Budapest, Hungary
 Phone: +36-1-372-2570
 Cell phone: +36-30-255-3111
 Fax: +36-1-372-2620
 URL: http://organ.elte.hu/farkas