Subject: Re: CCL:Gaussian Warning:"explicit consideration of 33 degrees of freedom as vibrations may cause significant error"

From: Georgina Aitken <g.d.c.aitken ~~ sussex.ac.uk>
Date: Fri, 01 Apr 2005 15:37:02 +0100

Dear Wenping Guo,

I have had a similar problem when I do frequency calculations on large 
floppy molecules in both G03 and G98.
Using the opt=tight or opt=verytight keywords can reduce the number of 
degrees of freedom that have low frequencies and using the 
freq=HinderedRotor keyword can also help. In most cases you can freeze out 
these vibrations by doing a frequency calculation at a low temperature e.g. 
10K.
However I would be interested to hear if you manage to get rid of the first 
warning message in the thermochemistry section of your output.

Back in november I posted the following question on CCL and had three 
useful responses to the post, all of which is summarised below:

Dear CCLers,

I am a PhD student and have been getting the following warning messages in
the thermochemistry section of my Gaussian frequency outputs for various 
divalent Pb-ligand complexes:

"Principal axes and moments of inertia in atomic units:
                           1         2         3
     EIGENVALUES --  4494.802316560.791727501.25369
           X            0.99998   0.00350   0.00560
           Y           -0.00289   0.99445  -0.10519
           Z           -0.00593   0.10517   0.99444
 This molecule is an asymmetric top.
 Rotational symmetry number  1.
 Warning -- assumption of classical behavior for rotation
           may cause significant error
 Rotational temperatures (Kelvin)      0.01927     0.01320     0.01155
 Rotational constants (GHZ):           0.40152     0.27508     0.24059
 Zero-point vibrational energy     647177.6 (Joules/Mol)
                                  154.67917 (Kcal/Mol)
 Warning -- explicit consideration of  23 degrees of freedom as
           vibrations may cause significant error "

Does this mean that the zero point energy correction is not reliable? What
can I do about it?

Thanks in advance,

Georgina Aitken

1.
Hi Georgina,
that means you have ltos of frequencies (23 of them) which are very small
and harmonic approximation may not be valid you can read more about it  at 
the G03 web site white papers:
http://www.gaussian.com/g_whitepap/white_pap.htm

some of these modes correspond to rotational motion of ligands (like 
hindered rotation)

If you optimized using the tight criteria some of those low frequenices 
will go away.  For large molecules there will always be some low frequency 
modes..

Shaji

2.
Georgina,

The warning ("explicit consideration of  23 degrees of freedom as 
vibrations may
cause significant error") means that 23 of the computed vibrational 
temperatures
are below some threshold value calculated by Gaussian to correspond to an 
ideal
gas. This threshold, for STP conditions, corresponds to vibrational 
frequencies
of around 625 cm**(-1), so if your low frequencies are only slightly below 
this
mark, there shouldn't be a problem with your zero-point correction.

How many degrees of freedom are in this system? Are you using constraints 
on any
of the atoms?

Ryan

3.
Georgina,

The second warning:

 "Warning -- explicit consideration of  23 degrees of freedom as
           vibrations may cause significant error"

is displayed whenever there are vibrational frequencies below a certain 
wavenumber.  The purpose of this warning is to remind the user that 
treatment of hindered rotations (such as those of the C-C bonds in ethane 
or propane) as "vibrations" may affect the thermodynamic calculations.

I don't think this error has any significant effect at 0K since these 
rotations are "frozen out" and hence really do act as vibrations.  At 
298.15K, however, there may be a noticeable (but still usually small) 
difference in heat capacity and entropy.

There is an option freq=hindrot which identifies the internal rotations and 
uses a different approximation which is usually better than treating these 
as vibrations.

--

As for the first warning:

"Warning -- assumption of classical behavior for rotation
           may cause significant error"

I've not encountered it before.  I suppose this means that the rigid rotor 
approximation is likely to be inaccurate, but I don't have a fix for that.

And even worse, this possibly could affect zero-point energy.  By how much, 
I have no idea.

--David Shobe, Ph.D., M.L.S.

--On Monday, March 28, 2005 11:37:17 +0800 
"=?gb2312?q?=B9=F9=CE=C4=C6=BD?=" <ybyygu ~~ yahoo.com.cn> wrote:

> Dear CCLers;
> I performed an freq calculation with G98 from a
> optimized structure.
> When finished, I found the following warnings in the
> output:
>
> Warning -- assumption of classical behavior for
> rotation
>            may cause significant error
>
> Warning -- explicit consideration of  33 degrees of
> freedom as
>            vibrations may cause significant error
>
> I am wondering what the warnings mean? Can I just
> neglect them, or how
> can I correct this situation?
>
> I would greatly appreciate any suggestion.
> Best wishes,
>
> Wenping Guo
>
> ------------------------------------------
> Path:     /China/Gansu/LanzhouUniversity/
> Address:  4006 ~~ physchem.lzu.edu.cn
>           ybyygu ~~ hotmail.com  # MSN
>           ybyygu ~~ yahoo.com.cn # Yahoo!
> -------------------------------------------
>
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Georgina Aitken

DPhil Student
University of Sussex
BN1 9QS
U.K