Subject: G94, Wrong Freq?

From: "Oscar N. Ventura" <[email-masked]>
Date: Thu, 15 Aug 1996 15:07:20 -0700

Dear CCLers,

In performing a DFT analytical frequencies calculation (UB3LYP/ 
6-311+G(d,p), forces and displacements converged to less than 5E-05) I 
found the following usual data about low frequencies (it is a 
hydrogen-bonded complex)

>Low frequencies ---  -30.7468   -0.0030    0.0077    0.0087    6.7016   
>8.4499
>Low frequencies ---   21.8223  136.3306  308.7007

However, just inmediately follows:

>******    1 imaginary frequencies (negative signs) ******
>                    1                      2                      3
>                   A"                     A'                     A"
>Frequencies --   -14.8965               136.3300               308.7004

While the second and third frequencies are coincident, the first is not. 
Although not so extreme, the same difference occurred with the MP2 
frequency calculation (41.38 vs 40.51), and the UHF one (12.8456 vs 
12.8388 cm-1).

Questions: (a) has anyone suffered the same problem?
	   (b) which of the two sets of info is wrong? (I could not
		get useful info from the eigenvectors)
	   (c) is there some ground to consider it a numerical problem?

Thanks for all the help,

		Oscar

----------------------------------
Prof. Dr. Oscar N. Ventura
MTC-Lab, C.C. 1157, 11800 Montevideo, Uruguay - 
http://bilbo.edu.uy/MTC-Lab.html
Tel:+598-2-941860 Fax:+598-2-941906/948396 e-mail: [email-masked]