Re: CCL:Bonds vs No Bonds in Minimized Structures

I just wanted to say thank you to all that responded to my query concerning
 GaussView's quirkiness about drawing bonds in minimized structures (see
 below for the full question).
 All of the answers were very helpful and all relayed essentially the same
 message:  just because a bond is not drawn does not mean there is no bond.
 GaussView apparently has a certain maximum bond length assigned to the N-N
 bond beyond which it arbitrarily decides not to draw a bond.  Certainly I
 suspected this to be true, but, as still a relative newbie at computational
 chemistry, I wanted to be sure.
 As always, some very useful information was conveyed, especially concerning
 the application of Bader's AIM, Weinhold's NBO, and ELF (electron
 localization function) theory to determine if a bond truly exists.  I look
 forward to employing these methods.
 Best regards!
 Gary W. Breton
 Chair and Associate Professor
 Department of Chemistry
 Berry College
 PO Box 495016
 Mount Berry, GA 30149
 "There's a light at the
 end of the tunnel, but it
 may be an oncoming train"
 ********************************************
 > From: Gary Breton <gbreton \\at// berry.edu>
 > Date: Wed, 14 Nov 2001 07:16:46 -0500
 > To: <CHEMISTRY \\at// ccl.net>
 > Subject: CCL:Bonds vs No Bonds in Minimized Structures
 >
 > Hello all,
 >
 > I have been conducting some geometry minimizations on some
 > 1,2-diazacyclobutane structures using Gaussian98 with the GaussView GUI.
 > When I minimize the parent compound (1,2-diazacyclobutane with N-H bonds)
 at
 > the RHF631G* level, everything looks fine.  If, however, I minimize at the
 > RB3LYP6311+G** level, a similar structure to the HF minimization results,
 > but GaussView does NOT SHOW a bond between the two nitrogen atoms.  The
 bond
 > has apparently not broken since the molecule adopts a geometry as if its
 > still there (i.e., in the HF minimization the N-N, C-N and C-C bond lengths
 > are 1.45405, 1.47272 and 1.53174, respectively, while in the B3LYP they are
 > 1.49052, 1.49351 and 1.53844, respectively).  Furthermore, if I start with
 > the HF structure and further minimize at the B3LYP level, the bond again
 > disappears!  If I start with the B3LYP structure and ADD a N-N bond and
 then
 > re-minimize, it again eliminates the bond.
 >
 > Is there any significance to the presence or absence of the bond?  Why
 would
 > GaussView not put a bond into the structure?
 >
 > Thanks and best regards!
 >
 >
 > Gary W. Breton
 > Chair and Associate Professor
 > Department of Chemistry
 > Berry College
 > PO Box 495016
 > Mount Berry, GA 30149
 >
 > "There's a light at the
 > end of the tunnel, but it
 > may be an oncoming train"