CCL: G09: Convergence Issues for H-Bonded Systems

 Sent to CCL by: Patrick Bultinck [Patrick.Bultinck#,#ugent.be]
 Just as a minor comment: drop that basis set. It does not render benzene planar
 for MP2, so I scare to think what it might be up to now.
 Second, I would get the Hessian from the fchk file, diagonalize that myself, see
 what that does and then compute the displacement eigenvector for the lowest
 eigenvalue and displace the geometry for minimal displacements along this vector
 and repeat the procedure. You could even think of some sort of grid of
 displacements and see if you could do some sort of line search along that.
 A near zero eigenvalue means that the energy does not change along that vector.
 Think of translation/rotation of the molecule as a whole. Check what your vector
 is like. And maybe it's just a zero direction: you may be looking at nice
 chemistry! Or some fragment may be so far out that you're seeing free movement.
 A minimal amount of algebra will tell.
 Patrick
 On 22 Sep 2011, at 17:25, "Sam Abrash sabrash]_[richmond.edu"
 <owner-chemistry|,|ccl.net> 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>
 >