Dear Milan

Thanks for reply.

I know the formula of adsorption energy but my questions are slightly different. 

Dear Mr. Verma,

if I understood your question correctly I would seuggest the following approach.

Generally in quantum chemistry we consider electronic energy differences between the most stable ground states of the single components (in your case, adsorbate, surface, and complex).

Therefore you should accurately find which spin configuration is the lowest in energy and then calculate the adsorption energy as usual.

Regarding your question on the ZPVE, I do not understand what you want to mean with "optimization can't be done, that would distort the ideal slab structure".

Nevertheless, to calculate ZPVE you calculate the vibrational frequecies (harmonic or anharmonic) and you sum up the contributions depending on the model potential you are considering, e.g. for an harmonic oscillator see

http://farside.ph.utexas.edu/teaching/sm1/lectures/node68.html

Dear Dr. Piccini

Thank you for your valuable input. 

My first question is clear now. However, my second question was about the following:

I am taking catalyst slab (monolayer) of hcp or fcc configuration in Gaussian 09 software with its ideal metal-metal bond distances etc. If I would go for optimization then the ideal hcp structure would be distorted and it won't represent the Metal(111) surface. Now, since I can't go for optimization and frequency (which would have given the ZPVEs), is there anything to calculate the ZPVE of that structure.?

Anyway, I'll go to the link given by you first........

Thanks 

Dear Mr Verma,

I am not a great expert in adsorption, but may be I can help you with optimization.

If I understood your correctly, the problem is that the structure can be distorted. Try opt=modredundant command. It will allow you to fix the bond lengths, angles and/or dihedral angles. You can fix the bonds you need and don't worry about them.

Hope it will help!

BR

Amina

Thanks Amina,

One curiosity, i.e. if opt=modredundant is fixing all parameters (e.g. bond length, angle, dihedral angle) then how would it optimize the structure?

Suppose, I have 12 atoms catalyst slab and all parameters are fixed then what is the role of optimization?   

Dear Mr Verma,

you don't need to fix all. For example, for some of my studies I have to freeze all dihedral angles. I assume you only want to fix some of the parameters to keep structure as ideal as possible.

BR,

Amina

A single layer of metal atoms would represent the catalyst rather poorly, I'm afraid. You need more atomic layers to emulate the bulk of the catalyst that has strong influence on the absorption, not only because it modifies the properties of the outermost layer of atoms, but also because it gets polarized by any ionic or dipole-like adsorbat.

Having added the inner layers (two at least, I think), you can freeze the positions of the inner-most atoms and allow the other to relax during the adsorption process. This would give you a much more reliable model of the real reaction.

Good luck :)

Marcin

Dear Dr. Marcin

Thanks for your valuable input. And, I would like to get your feedback on the following text.

I have done the adsorption for two types of surfaces (mono- and bi-layer) and finally, I'm going to see the adsorption energy difference. If the difference is ~1 kcal/mol then I'm going to perform the reactions on monolayer. Would this procedure be okay because I have very limited computational resources? 

Please respond to this....

If you have different suggestion then please write. 

Thanks again.

So I want to ask you about your surface. If its a thin film you are studying then what you did above is fine. But if you are trying surface chemistry with a slab model, you would need many more layers and also implement periodicity. usually you assume 3-4 layers of metal with the top two allowed to relax and the bottom layer fixed to simulate the remaining bulk below the surface (if its the structure is more complicated then you would need more layers). If you have a thin film and it distorts on optimization than either the level theory + functional isn't suited to describe the metal or its unstable as a thin film. I would suggest trying to use a planewave code. VASP is very user friendly but requires a license while they are other free software like quantum espresso and ABINIT.