CCL: DFTB geometries

From: "Richard Leo Wood" <woodx278::umn.edu>
Subject: CCL: DFTB geometries
Date: Tue, 25 Sep 2007 12:05:42 -0400

 Sent to CCL by: "Richard Leo Wood" [woodx278!A!umn.edu]
 Hi all,
 I'm encountering a problem with geometries obtained using DFTB and I am
 wondering if anyone else has encountered a similar problem.
 I have a series of 12 molecules that I am using to do a validation study.  All
 of the molcules contian nitrogen and seven of them contain nitrogen and sulfur,
 as well as carbon, hydrogen and oxygen.
 The problem I am having is that for all seven of the sulfur containing
 molecules, I get weird geometries, if they even converge (two of the seven won't
 even converge).  What I mean by weird is that some of the bond lengths are
 REALLY long (like C-C bond of 4 Angstroms or more).
 I'm wondering if anyone else has encountered this "problem" using DFTB
 and if there is perhaps a work-around to it.
 TIA,
 Richard