Freeh - linear molecules

[Ian K]

I have an optimised linear molecule that I ran in c1 symmetry, and I've run aoforce, but I'm not entirely sure whether freeh is going to give me correct results for the thermodynamic properties, and what value of Sigma I should provide in this case.

Could anyone comment on what modifications or special input (if any) I should provide in order to get correct values, or whether this is possible with the model freeh uses?

[Arnim]

Hi Ian,

Sigma denotes the number of ways the molecule can be oriented which are indistinguishable
from each other. For a linear molecules it depends, if you have a mirror plane in the
middle or not. For molecules with C_(inf)v Sigma=1, for D_(inf)h Sigma=2.
 
You can find some more details on the rotational partition function and what they call 'External Symmetry Number' on this page:
http://cccbdb.nist.gov/thermo.asp

Cheers,
Arnim

[Ian K]

Ah, thanks, that's a helpful way to think about it.

But then wouldn't I have to perform some sort of correction for the difference between the rotational partition function for linear and nonlinear molecules, I assume, related to 2kT(pi) and a term related to the reduction of the inertial terms?

freeh gives the two different formulae for the rotational partition function, but it's not clear to me whether the difference is something I have to work out how to include myself or something that is produced 'automatically' by the way Turbomole obtains the result.

[Arnim]

The decision, whether the formular for the linear molecule is used, is made by freeh.
This can be done straightforward by checking, if one of the moments of inertia is zero (maybe better 'numerically zero').

So, here is no input required. The correct partition function should be used automatically.

[Ian K]

Excellent, thanks again.