How to calculate dG

[sonic]

hey 
I just hit upon this turbomole forum (google). I'm currently teaching myself how to calculate with gaussian03 and turbomole (but I rather need to learn the theory of this whole stuff).
so what I do is generally calculating geometries, energies and frequencies (DFT/b3lyp) (on my own). I'm just interested in learning it.

I know how to calculate the reaction enthalpy, it is simply: [Energy(Products) - Energy(Educts)]
I got good results for a reaction where two Monomers react to a Dimer ( 2A ---> B )
so I subtracted the Energies: [ B - (2*A) ] and recieved the reaction enthalpy ΔH.

My question:
Is is possible to calculate the "Gibbs free energy ΔG" ? At least I heard about that. I think I probably have to use tabulated entropy values, right?
Can someone please explain me how to do that?

p.s.
Can anybody recommend me a computationalchemistry-book where the basics are explained (Whats SCF, Hatree-Fock, DFT and so on) [for dummies if procurable - german books preferred]

[FKoehler]

Hi,

Is there any particular reason, you can't directly use the definition of
Gibbs free energy?

G = H -TS,

If you compare this with the output of freeh, you'll see, that
entropy = (energy + RT - chem.pot.) / T, and
enthalpy = energy + RT

==> G = chem.pot.

So you could also just use the chem.pot. from the output. Which miraculously ;-)
is the same as the IUPAC Definitions:

Δ_rG =  Σ_i(ν_iμ_i)

see
>http://www.chemgapedia.de/vsengine/vlu/vsc/de/ch/13/vlu/thermodyn/gleichgewicht/chem_ggw.vlu/Page/vsc/de/ch/13/pc/thermodyn/gleichgewicht/chemreak_ggwkherltg.vscml.html

Best regards.

[sonic]

Hi,

Is there any particular reason, you can't directly use the definition of
Gibbs free energy?

G = H -TS

I need the entropy value :)i didnt have it.

what do you mean with "freeh"?
where in the logfile (calculated with gaussian) can i get the values for the chem. pot. ? i didn't find it.

i only found these:
Zero-point energy, Zero-point correction, Thermal correction to Energy, Thermal correction to Enthalpy, Thermal correction to Gibbs Free Energy, Sum of electronic and zero-point Energies, Sum of electronic and thermal Energies, Sum of electronic and thermal Enthalpies, Sum of electronic and thermal Free Energies.

thanks!

[FKoehler]

Hi,

Is there any particular reason, you can't directly use the definition of
Gibbs free energy?

G = H -TS

I need the entropy value :)i didnt have it.

what do you mean with "freeh"?
where in the logfile (calculated with gaussian) can i get the values for the chem. pot. ? i didn't find it.

freeh is the Turbomole program that will print out thermochemical values after a frequency calculation.
Considering this is the forum for the quantum chemistry program TURBOMOLE, I assumed you are using TURBOMOLE, not Gaussian ;-)
A Gaussian mailing list or forum probably would be more appropriate.

Anyway:

i only found these:
Zero-point energy, Zero-point correction, Thermal correction to Energy, Thermal correction to Enthalpy, Thermal correction to Gibbs Free Energy, Sum of electronic and zero-point Energies, Sum of electronic and thermal Energies, Sum of electronic and thermal Enthalpies, Sum of electronic and thermal Free Energies.

Look down 2 more lines, There is an entry S in Cal/Mol-Kelvin.
The total is the entropy you are looking for.

And if you are looking for Gibbs free energy, you can just use the "Sum of electronic and thermal Free Energies".

Regards, Felix