Subject: SUMMARY: One negative eigenvalue - No imaginary frequency!!??

From: Carlos Silva Lopez <csilval(at)uvigo.es>
Date: Thu, 26 Aug 2004 20:51:21 -0700

Hi CCLers,

I have already received a good number of suggestions, I would like to 
thank specially Per-Ola Norrby, J. Fox (gaussian support), Steve 
Williams, Shaji chempath and Anastassia Alexandrova (I hope I am not 
forgetting anybody).

Some of the suggestions doubted my skills to reach a stationary point on 
the PES ;-) actually that had been my first guess and I tightened the 
convergence criteria of the SCF and the optimization to the maximum, I 
also used an uncommonly fine integration mesh. So, yes, I am at a 
stationary point (well, the structure is, to be rigourous). Actually I 
re-started the calculation until I got a converged structure with null 
forces (at the precision level of the gaussian output, of course) in the 
frequency job.

Other suggestions were to employ more robust methods for this particular 
serching (QST2,QST3,Nudged Elastic Band). I tried the first two methods 
with no better luck.

Another person pointed out that gaussian might sometimes interchange the 
last rotation/vibration with the first vibration. I am not sure about 
how to check this, but it looks like a possibility due to my lasts 
trials (see below).

Several suggestions pointed that my calculation was not reaching close 
enough to the TS or reaching a shallow minimum instead. As I said I am 
defenitely at a stationary point (from the ultra low forces and 
displacements I got) but if I take this converged structure and run it 
to converge to a minimum....surprise!!!! it goes off towards the "real" 
minimum of the products of the nucleophilic attack.

In a nutshell, I am even more clueless than before even though I have 
received really nice and welcomed help from several netters. If nobody 
has any further idea about what's going on here I will eventually 
re-converge everything within a continuum dielectric method to see if it 
counterbalances the anionic charge state and everything goes better.

Thanks so much to everybody supporting this list!