I want to plot enthalpy with pressure concentration from DFT for high pressure phase change study. DFT gives U, P and V values but H=U+PV is valid for gaseous phase. So how does calculate the enthalpy for solid phase ?
I am using VASP code. I have optimized structure and taking that structure I have calculated different E0 for different pressure by varying volume.
Mr. Sharma,
Please find as an attachment DFT-calculations by Gaussian (www.gaussian.com) of crystals. Using as input parameters the crystal structures you can compute the thermodynamic parameters, including enthalpy at different T, P, including isotopes. Furthermore, there is possible to combine all the calculations within the frame of a multi-step job.
As you could recognize from the attachment each of the additional jobs is determined separately, where you should only specify the parameters for example "charge and spin
350.0 3.0", meaning in this case additional step at T = 350.00 K and P = 3.0 Atm.
In "default", using the keyword "Freq", you have obtained thermodynamic data at T = 298.150 K and P = 1 Atm.
Given that you could carry out computational prediction of enthalpy for a crystal for example, which is solid-state phase at various experimental conditions and to plot your relationship H = f(T, P and/or isotopes).
Those computations can be separately performed as well. Given that you can correlate, for example, crystallographic data of a crystal, obtained at different T and P versus computational geomery optimizations by DFT or thermodynamic properties.
Each version of "Gaussian" has manual, describing details to each computational steps. Manuals can be obtained from the web-site shown above, too.
Who told you that H=U+PV is not valid for solids? It's a general thermodynamic definition (transformation)! Just the pressures are much higher than in gases ;-).
The only complication is that you have to find a reliable equation of state (EOS) U=U(V) or U=U(p) and from that p=p(V)=-dU(V)/dV, e.g., via Murnaghan's, Vinet's or Birch's EOS by fitting calculated data (around the equilibrium volume) to the EOS in order to determine the key parameters occurring in the EOS (usually E_0, V_0, B_0 and B'_0=dB_0/dp(V=V_0)). Once you have the fit parameters (the analytic form of the EOS) you can then determine p(V) (or V(p)) and hence H=U+PV as a function of V or p.
You can then plot H(p)=H(V(p)) for each phase and the crossing points give you the transition pressures. Knowing p(V) you can look at the corresponding pressure which volumes the phases take and hence how large the volume change is at the transition. Just take care that you compare always the same number of atoms or have normed everything to the same number of atoms (energies, volumes).
Thank You Dr. Jürgen Furthmüller for discussion.
I will apply this.
I recommend you to read this paper:
http://ijens.org/Vol_18_I_05/180505-3737-IJMME-IJENS.pdf
Best regards
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