CCL:G: Relative Stability of species.

[Peter Kraus <peter.kraus/./geeksonbikes.net>]

Dear Vimal,

it looks as if you're conflating two issues: first is whether a point on your potential energy hypersurface (i.e. your geometry) is at a minimum in energy - this, at least for small covalent (single reference) compounds should be true as long as your geometry optimisation converges (the "four YES's" in Gaussian). It is good to check it's a minimum by doing a frequency calculation after the optimization step. If there are no imaginary frequencies, you're good to go.

As for "stability", the question is - at what temperature? You could compare the absolute energies of your three compounds directly, as long as they have the same elemental composition. If not, you have to calculate the heat of formation by difference from the correct number of infinitely separated atoms. However, even at 0 K, you will have zero-point effects, which you can estimate by the vibrational frequencies. They might be large enough to shift the balance of stability around. Then there are the more important thermal effects, where the "tutorial" you mentioned comes in - the Gibbs energy is a function of both enthalpy and entropy, and entropy is related to partition functions, which can be calculated from the vibrational frequencies and rotational constants. If you need help with that, you can always contact the folks at Gaussian Inc. directly, after all you're paying them a lot of money to use their software. I've found them very helpful.

Best regards,

Peter

On Tue, 2 Oct 2018 at 21:41, Jeya Vimalan jeyavimalan2k-*-gmail.com <owner-chemistry^ccl.net> wrote:

Dear all,

I have the following question that i need your help to get

it cleared.

I have three molecular species that differs in the composition. 

Say for example, TiCl3, TiCl2, and TiCl4. 

From the vibrational frequencies, i can understand if they 

are stable in the PES (potential energy surface). But

are there any way that i can determine the relative stability

of these three species. 

There is a tutorial in G09, that describes the thermochemistry

to determine the free energy of molecules, but i see that certain

parameters are taken from literature and it is not available

for the complete set of periodic table. 

Can any one point me some tutorials/work outs that have been made

earlier for a novice user. 

Thanks in advance.

Vimal.

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