Subject: G: Wrong Freq - insufficient convergence

From: "Stefan Fau" <[email-masked]>
Date: Fri, 16 Aug 1996 22:33:38 MDT

Hi,

here are my thoughts concerning the difference between frequencies and 
Low-frequencies. If I tell something wrong, feel free to correct me!

(a),
we encounter the same phenomenon, if we calculate molecules with very flat 
PES's. An extreme example is H3C-Be-CH3 (D3d) at the MP2/6-32G(d) level. When the 
structure is converged to about half of the standard limits, the biggest of the 
six "Zero"-frequencies is 52 cm-1 and the next Low-frequency is 55 cm-1. I do 
not recall the corresponding value in the complete list of frequencies, but it 
was different. However, after reoptimization of the molecule with the very-tight 
limits, we got very good agreement between frequencies and Low-frequencies. The 
first 6 Low-frequencies were between -1.36 and -0.00 and the next three were
18.11, 201.01, 201.01. The corresponding frequencies match perfectly.

(b),
we assume that the frequencies and Low-frequencies are calculated with 
different algorithms, which include different approximations and should both be 
correct with a perfectly converged structure. But, as nothing is perfect, in 
practice the results differ a little bit.

(c),
the softer the PES around your structure is, the greater are the differences of 
the two algorithms. Therefore we consider it a problem of the convergence 
criteria. We always got good agreement between frequencies and Low-frequencies, 
when we reoptimized the structure (e.g. opt=(readfc,tight)).

As the standard convergence criteria are made for standard molecules, problems 
can arise with very soft molecules. Therefore I use the agreement between 
frequencies and Low frequencies as an additional criterion for convergence. I 
arbitrarily chose a maximum difference of 0.01 cm-1 between corresponding freqs 
and Low-freqs as sufficient and also look, if the ratio of the biggest 
"Zero"-freq and the first "real" freq exceeds (-)25.

Maybe somebody can comment on this?

Stefan

Stefan Fau,               [email-masked]

FB Chemie der Philipps-Universitaet Marburg,
Hans-Meerwein-Str.
D-35032 Marburg