Subject: Re: CCL:Is the scale available for B3LYP/LanL2DZ FREQ calculation?

From: Marcel Swart <[email-masked]>
Date: Wed, 30 Aug 2000 09:06:00 +0200

>Hi, everyone,
>
>I tried some metal clusters' G98 jobs in these days. I cannot find the
>frequency scales for LanL2DZ basis set. Could somebody have the scale
>for B3LYP/LanL2DZ? If possible, please tell me the reference.

For once and for all.

The scaling factors are from ancient history when it was known that 
RHF gave rather bad (i.e. low) frequency values. Some people used 
scaling factors to (ad hoc) increase these values, but these factors 
should be treated with special care. Since they are averaged over 
some set of molecules, it doesn't necessarily mean that the molecule 
you are treating right now exhibits the error in the same amount. 
This is especially true for DFT calculations which normally give very 
good agreement with experimental frequencies.

In my opinion these factors should be ignored all over.

Therefore the answer is: it is one (1.0000).

M. Swart

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From: Patrik Johansson <[email-masked]>
Date: Sun, 30 Jul 2000 13:53:08 +0200

Well, I do strongly disagree with the opinion expressed by Marcel Swart.
Still, RHF calculations are the only methods practically useful for
large molecules and therefore these ancient(?) scaling factors to
_lower_ the to _high_ frequencies obtained are most useful. I wouldn't
call a paper like Scott & Radom JPC 1996 an ancient one...

And, of course no one uses _anything_ without verifying that it actually
works - that is, gives a good comparison with experiment.

And if we go back to the original question - I've used the scaling
factors for HF/6-31G* (0.89) for calculations with HF/LanL2DZ on Ag and
the rest of the molecule just HF/6-31G* and it worked fine against my
experimental Raman data. Therefore you could try the B3LYP/6-31G* one -
should be 0.98 or better - I don't remember right now.

Patrik

Marcel Swart wrote:
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---

From: Wolfgang Roth <[email-masked]>
Date: Wed, 30 Aug 2000 10:28:12 +0100

Hello,

>you are treating right now exhibits the error in the same amount. 

I completely agree with that Marcel, but,

>good agreement with experimental frequencies.

This statement is useless without any given proof. The ratio between
experimental and theoretical/calculated frequencies may be closer to one
than for HF calculations but this depends on the chosen functional and
basis set... 

>In my opinion these factors should be ignored all over.
I agree to a certain point: 
To interpret a vibrational or vibronic spectrum it is very useful to work
with scaling factors to assign all the signals. Tabulated values might
provide a useful hint to start with. 

But I do not believe that a scaling factor obtained from a fit to the G2
set of molecules will be a reliable estimation for metallo-organic compounds.

To me, the ratio of experimental and calculated vibrational frequencies is
a good measure to evaluate theoretical methods: For MP2/6-311G(d,p) and a
number of aromatic molecules, the ration for the modes below 2000 cm^-1 is
about 1 while it is 0.95 for the x-H stretch vibration above 3000 cm^-1.
The overall "scaling" factor would be around 0.99 but the deviation between
exp. and calc. for the latter modes makes the assignment difficult in some
cases. However, at this level of theory Wilsons/Varsanyis mode 4 turns out
to be completely wrong...

Your assumption of a scaling factors with is one for all cases implies that
theory is -at least- more exact or reliable that the experimental spectrum.

>Therefore the answer is: it is one (1.0000).
Adding decimals to the cipher/numeral does not increase the 'correctness'
of that value...

Regards
Wolfgang Roth

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