CCL: G09: Convergence Issues for H-Bonded Systems

From: "Sam Abrash" <sabrash[a]richmond.edu>
Subject: CCL: G09: Convergence Issues for H-Bonded Systems
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