Author Topic: Gibbs Free Energy and VASP  (Read 6693 times)

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Offline markmick

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Gibbs Free Energy and VASP
« on: August 9, 2016, 06:25 »
I've successfully performed binding energy calculations with VASP, but I'm stumped with Gibbs free energy.  Based on an article I'm studying, Gibbs Energy for an adsorbed hydrogen is calculated by:

ΔGH = ΔEH + ΔEZPE - TΔS

Some of these variables I understand, others I don't:

* T is temperature in Kelvin (Setting this in INCAR I believe)
* ΔS is entropy, which I believe is pulled from OUTCAR
* ΔEH  is the binding enegy, and I know how to do this one
* ΔEZPE (Zero-point-energy).  This one has me stumped. 

ΔEZPE  = ΔEnH - ΔE(n-1)H - 1/2 EH2

ΔEZPE has something to do with meV and THz values pulled from OUTCAR.  My INCAR is set with:

NELMIN = 4
ISMEAR = 0
IBRION = 7
ISIF = 0
LEPSILON = .TRUE.
NSW = 1
NWRITE = 3

and this produces an OUTCAR with final values like:

63 f  =    0.665149 THz     4.179253 2PiTHz   22.186976 cm-1     2.750835 meV

How to translate this OUTCAR data into the values for ΔEZPE elude me.  I assume just use the meV value?   Thanks for any assistance.  I'm accustomed to Gibbs from Thermodynamic calculations, but using VASP for this is a new frontier.

Sincerely,
Mark




 

Offline Anders Blom

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Re: Gibbs Free Energy and VASP
« Reply #1 on: August 9, 2016, 08:51 »
You will probably have more luck getting this question properly answered at the VASP forum than here.

Offline zh

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Re: Gibbs Free Energy and VASP
« Reply #2 on: August 21, 2016, 10:30 »
For an A2 diatomic molecule, the zero point energy is 0.5* h *v, where h is Planck's constant, and v is the vibration frequency of this molecule.