CCL:G: G09: Convergence Issues for H-Bonded Systems

 Sent to CCL by: Sten Nilsson Lill [stenil||chalmers.se]
 Sam,
 in similar cases I've been able to reach convergence by using the keywords
 scf=novaracc and opt=(tight,newton). To those opt-keyword you could also add
 calcfc as you mention.
 Another aspect of this is the sensitivity of the M06 functionals to the choice
 of integration grid. It's recommended to use a large grid so if you are using
 Gaussian you can try Grid=ultrafine. Remember that it will be more costly and
 that you need to do all your calculations with the same grid settings when
 comparing energies.
 You can read more on this in Houk's JCTC paper from 2010: http://pubs.acs.org/doi/abs/10.1021/ct900639j and some
 references therein, for example some work by DiLabio and Johnson.
 Hope that helps,
 Sten
 Ph.D. Sten Nilsson Lill
 Dep. of Chemistry
 University of Gothenburg
 SE-412 96 Gothenburg, Sweden
 e-mail: stenil|-|chem.gu.se
 Phone: +46-31 786 9103
 Fax: +46-31-772 3840
 ________________________________________
 > From: owner-chemistry+stenil==chem.gu.se|-|ccl.net
 [owner-chemistry+stenil==chem.gu.se|-|ccl.net] On Behalf Of Sam Abrash
 sabrash]_[richmond.edu [owner-chemistry|-|ccl.net]
 Sent: Thursday, September 22, 2011 5:25 PM
 To: Nilsson Lill, Sten
 Subject: CCL: G09: Convergence Issues for H-Bonded Systems
 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!
 Samhttp://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp-:-//www.ccl.net/chemistry/sub_unsub.shtmlhttp-:-//www.ccl.net/spammers.txt