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Parrinello-Rahman (NpT) dynamics with Langevin thermostat

The Parrinello-Rahman dynamics is currently available only in connection with Langevin thermostat 6.62.5. The geometric constraints and metadynamics are not supported in the current version of VASP. See Sec. 6.62.6 and 6.62.5 for brief description of the Parrinello-Rahman dynamics and Langevin thermostat, respectively.
  1. Use the same setup as for Langevin dynamics in NVT ensemble (see Sec. 6.62.2) but set ISIF=3 to allow for the cell volume and cell shape variations. At the moment, dynamics with fixed volume + variable shape (ISIF=4) and fixed shape + variable volume (ISIF=7) are not available.
  2. Use LANGEVIN_GAMMA_L to set friction coefficient for lattice degrees of freedom (see 6.62.3)
  3. Set mass for the lattice degrees of freedom using the parameter using the parameter PMASS 6.62.3
  4. Optionally, external pressure (in kB) can be defined using the parameter PSTRESS
Note that the temperatures listed in the file OSZICAR are computed using the kinetic energy including contribution from both atomic and lattice degrees of freedom. The external pressure for a simulation can be computed as one third of trace of stress-tensor corrected for kinetic contribution listed in OUTCAR. This can be achieved e.g. by using the following command:
grep "Total+kin" OUTCAR| awk 'BEGIN {p=0.} {p+=($2+$3+$4)/3.} END {print "pressure (kB):",p}'

IMPORTANT: In Parrinello-Rahman dynamics, components of stress tensor are used to define forces acting on lattice degrees of freedom (see Ref. [79,80] for details). In order to achieve a reasonable quality of sampling (or even to avoid numerical problems), it is essential to eliminate Pulay stress. Unfortunately, this usually requires rather large value of ENCUT. The setting with PREC=low frequently used in NVT MD is not recommended for molecular dynamics with variable cell volume. For more details on the Pulay stress see 7.6.

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