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

[Arne Dieckmann <adieckma _ googlemail.com>]

 Sent to CCL by: Arne Dieckmann [adieckma . googlemail.com]
 I would not consider -1.9 wavenumbers an imaginary frequency. Have a look at the
 Gaussian manual and whitepapers, there is a section about this issue.
 Cheers,
 Arne
 On Sep 22, 2011, at 8:25 AM, Sam Abrash sabrash]_[richmond.edu wrote:
 >
 > 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>
 >