From owner-chemistry@ccl.net Thu Sep 22 11:42:00 2011
From: "Sam Abrash sabrash]_[richmond.edu" <owner-chemistry^_^server.ccl.net>
To: CCL
Subject: CCL: G09: Convergence Issues for H-Bonded Systems
Message-Id: <-45493-110922112540-27673-9hPHMW+QA8/i/01J/WeMKw^_^server.ccl.net>
X-Original-From: "Sam   Abrash" <sabrash[a]richmond.edu>
Date: Thu, 22 Sep 2011 11:25:36 -0400


Sent to CCL by: "Sam   Abrash" [sabrash_+_richmond.edu]
Hi Folks,

I have a job involving hydrogen bonding of 3 acetylenes to the benzene cation.  I'm having trouble with the geometric convergence.  I was able to get the system to converge with normal convergence criteria, opt=calcall, and MaxStep=5, but then I got an imaginary frequency of -1.9 wavenumbers.  

Subsequently, I changed to Opt=tight, tried both calcfc and calcall, and have steadily reduced MaxStep to 1, but it still won't converge.  The force constant criteria have been met, but the problem is the displacement criteria.  Both this observation and looking at the structures with JMOL show that the problem is finding the minimum in a very shallow potential.

Two questions.  First, is the 1.9 wavenumber imaginary frequency real or an artifact?  Second, any advice on how to force the system to converge?  Model chemistry is M062X/6-311++G**.

Thanks!
Sam