How we can obtain the entropy (TS) and the potential value (U) to get Gibbs free energy

[manoj_wijesingha]

Hello Colleagues,
I need to apply the Computational Hydrogen Electrode (CHE) model for the electroreductions. Therefore, I should calculate the Gibbs free energy by using this equation, G = E + ZPE - TS + U. E is DFT energy of the calculation, and we can get ZPE from the frequency calculations (sum of all the energy of all modes (minus 3 for translation)).

I have two questions regarding this:
The first one is that how we can calculate the entropy, TS, by using the frequency calculation output data, and can I apply this equation to compute the TS values, T*S = R * ∑[ν * ln(ν/T)], v- the vibrational frequency, R is the gas constant, and T is the temperature in Kelvin?

The other one is how we can get the value for the potential, U, to calculate the Gibbs energy.

I have attached a file of all vibrational frequency modes, and could you please explain for me to calculate the entropy and the potential with details by using this file.

Thank you.

[alex]

Hi,

the U in your equation is the internal energy of your system as defined by the first law of thermodynamics. Nothing to do with potential(s).

Cheers,

alex

[manoj_wijesingha]

Hi Alex,
Thank you for your response. So, how can I calculate internal energy (U) of the system, for example CO2 reduction,
CO2 + 2(H+ + e-) ---> CO + H2O ?
How can I get T*S value after frequency calculation for CO2 reduction (I had attached a file of all frequency modes, could you please explain using this file)?
Further, I did not change the temperature to room temperature (298.15 K or 300 K) for the frequency calculations (I use the default value, TEBEG = 0.0; TEEND = 0.0). Should I change the temperature in the INCAR file (TEBEG = 298.15; TEEND = 298.15) because of entropy calculation (T*S)?
Thank you.

[marie-therese.huebsch]

Hi,
It is not so trivial to compute Gibb's free energy. I think you need an advanced molecular dynamics method. Have a look at this lecture: Advanced methods of molecular dynamics | VASP Lecture.

I hope an experienced user will help you out

Marie-Therese

[alex]

Hello again,

if it comes to understanding the quantities you are after I found 'Thermochemistry in Gaussian' by Joseph W. Ochterski, https://gaussian.com/thermo/ very instructive. Beware, you have different number of degree of freedoms with solids.

How to get from a frequency calculation to S and G?
Nowadays I'd go for Phonopy or similar software as suggested by info/resource/

Hth, alex

[manoj_wijesingha]

Hi Experts,
Thank you so much, and appreciate you guys, Alex and Marie. I use phonopy for phonon calculation, and we can get the entropy by phonopy. However, I needed after frequency calculation by VASP, how we can use frequency values to calculate entropy (TS) except phonopy. One more thing, can we use vaspkit to get ZPE and entropy (T*S)? I have attached a file regarding this.
Thanks.

[alex]

If you want to get deep into that, read the book by Wilson, Decious and Cross. ;-)

[manoj_wijesingha]

Thank you Alex.