Dear all,
What is the difference between Car- Parrinello MD (CPMD) and varriable cell CPMD (vc-cp) in Quantum Espresso? Does it mean that the simulation is performing in NPT enssemble instead of NVT, for instance?
In the traditional Car-Parrinello Molecular Dynamics (MD) simulations, the unit cell volume and shape are fixed and one calculates the atomic positions and the self-consistent (Kohn-Sham) wave functions through a MD simulation. Of course, in such simulation both the unit-cell volume and shape parameters enter the energy functional as parameters, which however do not vary in the course of the MD simulation (they are static variables and not dynamic ones). In the Variable Cell MD simulations, as the name indicates, the volume and the shape of the unit cell are dynamical variables and are determined under a given externally-applied constant pressure. Such simulations are experimentally more relevant in that experimentally one fixes the external pressure and temperature -- both intensive thermodynamic variables, as opposed to the volume, which is an extensive thermodynamic variable. Such simulations are very relevant to theoretical investigations of structural phase transition (brought about by change in the temperature and the applied external pressure) at which both volume and shape of the unit cell can change. Incidentally, VCMD is also applied in the simulation of liquids, where shape of the simulation cell is essentially arbitrary, whereby one can simulate the applied external pressure by considering the volume of this cell as a dynamical variable.
Three most cited publications regarding the VCMD technique are: (1) HC Andersen, J. Chem. Phys. 72, 2384 (1980), (2) M Parrinello, and A Rahman, Phys. Rev. Lett. 45, 1196 (1980), and (3) M Parrinello, and A Rahman, J. Appl. Phys. 52, 7182 (1981).
Thank you very much dear Behnam. Did you have some experiences with cpmd simulation to investigate a chemical reaction?
You are welcome.
The answer to your question is no. But there are plenty of published studies around, using VCMD, such as those by Wentzcovitch et al. (Phys. Rev. Lett. 70, 3947 (1993)), and Souza and Martins (Phys. Rev. B 55, 8733 (1997)), to name but two.
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