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Re: How to calculate Gibbs-free energy, enthalpy and entropy change in firefly?


Dear Gensz,

Enthalpy can be calculated with the following formula:

Enthalpy = Electronic energy + Zero point energy + Vibrational energy + Rotational energy + Translational energy + p*V

The TOTAL ENERGY in ENERGY COMPONENTS is only the Electronic energy. The other energies which (together with the Electronic energy) make up the enthalpy can be calculated by a vibrational analysis. For this, you need to optimize the molecular geometry with RUNTYP=OPTIMIZE and then perform a vibrational analysis on the found geometry with RUNTYP=HESSIAN.

As I don't know what kind of system your investigating, I can't comment on whether you can perform geometry optimizations on it or not. However, an optimization followed by a vibrational analysis is AFAIK the >only< way to get the thermochemical data you'd like to obtain.

Hope this helps.

Kind regards,

On Mon Feb 13 '12 8:21am, gensz wrote
>Hello there, I am confused on how to interpret the output file in firefly, I just want to know that, is the TOTAL ENeRGY in :ENERGY COMPONENTS: for the output file is equal to the Enthalpy?...

>since I worked on radicals I can't use runtyp=optimize for mp2, therefore i'll only use energy and it does not give me any information on Gibss-energy, Enthalpy and Entropy...
>Does the information in the HF calculations giving those reaction energies Gibbs,enthalpy and entropy is used for calculating the change in Gibbs-energy, change in Enthalpy and change in entropy?

>Pls help..I'm really confused.

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