pressure and temperature

[Subhodip_Chatterjee]

Hi all,
I'm new in CASINO and I'm trying to learn by going through the manual. I want to know that can pressure and temperature be introduced in qmc calculations using CASINO??

Best Regards,
Subhodip

[Neil Drummond]

Dear Subhodip,

VMC and DMC are zero-temperature methods that work in fixed simulation cells.

If you want to calculate the equation of state of a solid then you need to calculate the DMC energy as a function of cell volume E(V). The static-lattice pressure at zero temperature can then be evaluated as p=-dE/dV. Typically this is done by fitting a model equation of state (e.g., Vinet or Birch-Murnaghan) to your set of E(V) points.

If you want to study finite temperatures then you need to add in the Helmholtz free energy of phonon vibrations to each of your static-lattice energies. These would normally be taken from DFT phonon calculations. You then have F(V,T), from which you can find the temperature-volume-pressure equation of state as p(V,T)=-dF/dV.

One can also calculate the pressure more directly but usually less accurately from QMC kinetic and potential energies using the virial theorem.

Best wishes,

Neil.

[Subhodip_Chatterjee]

Thanks Neil...