At 09:57 AM 2/15/01 +0000, you wrote:
>Dear all,
>
>in my calculated frequencies for a uranium containing closed-shell molecule,
>I get one imaginary frequency which is doubly degenerate due to symmetry.
>Now, while it is perfectly possible that these are 'real' because the
>molecule
>would have a lower symmetry in its ground state, I am still wondering
>whether this is, in fact, noise, given that the calculated frequencies are
>only
>13.0i cm^-1.
> Thus, my questions are: What is the accuracy of the finite-difference ADF
>frequencies for very floppy modes? Can calculated frequencies of 13i cm^-1
>still be noise? Any further comments on the subject?
> I am using scalar ZORA, INTEGRATION 8, default values for 'frequencies',
>PW91, frozen core ZORA basis V, cartesian coordinates, D5 symmetry.

Dear Georg,

Happy Valentine ! The imaginary frequencies you got are so small that it
might just be due to noise, as you suspected. I had similar problems
before. What I did was to increase the INTEGRATION from 6, to 8 and to 10.
It turns out that my small imaginary frequency goes from small imaginary to
positive when using INTEGRATION=10. Therefore for floppy molecule you
probably need to use 10 and also a tight convergence criteria for geometry
optimization (e.g. 1*10^-5), with a very tight convergence criteria
(10^-10) for SCF iteration as well.

Best regards,

Jun

P.S. I would check the low-frequency vibrational mode of a floppy molecule
using a method that has analytical 1st- and 2nd-order energy derivatives.

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Jun Li, PhD, Research Scientist
Department of Chemistry Tel: (614)292-5227 (Office)
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Received on 2001-02-16 15:44:27