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Dear Alex,

One more question. In my frequency calculations I find some soft vibrational modes that significantly affects the calculation of partition function using harmonic oscillator approximation. I read some where that one has to use other approximations e.g. hindered rotor but I could not find a good reference to understand what is a hindered rotor approximation and how I can use it to estimate the k for a reaction over a catalyst. I also do not know which modes should e considered as a soft mode! Do you know any good reference book about estimation of reaction rate constant? I have found some article like P. Y. Ayala and H. B. Schlegel, J. Chem. Phys. 108 2314 (1998) but I need a more fundamental understanding of it.

Regards
Abas

Hi,

check here http://www.gaussian.com/g_whitepap/vib.htm
and there
http://www.gaussian.com/g_whitepap/thermo.htm
for starters.

Cheers,

alex

Dear Alex,

I have tested what you suggested. It seems that it is not working. I moved atoms and after re optimization the structure is the same as before. I read your comment somewhere in the forum "It is not necessary to have imaginary modes. They mostly show up because of you are doing numerical derivatives (noise). If you want to make it perfect, you have to set up a projection matrix which removes modes which are related to translation of the entire molecule."(http://cms.mpi.univie.ac.at/vasp-forum/ ... php?4.8987)
Do you think setting up a projection matrix works for my system? If yes, how can I set it up? I searched a lot in VASP manual, I could not find any thing useful.

Regards
Abas

Dear all,
can anyone tell me how to extract heat capacity from the obtained frequencies?

Thank you in advance