Molecular dynamics calculations

To run a basic molecular dynamics calculation perform the following steps:

• Choose a POSCAR containing a large enough super cell.
• If a continuation run is performed copy CONTCAR to POSCAR or possibly deliver initial velocities in the POSCAR file. They are written after the Wycoff positions in an own paragraph. If no initial velocities are provided random velocities are assumed at the beginning of the calculation. This is fully ok but the user should be aware that due to the initial random velocities the trajectories obtained from different calculations are difficult to compare.
• Set main INCAR tags:
  • IBRION=0: Molecular dynamics calculations are enabled by setting the IBRION tag to 0.
  • POTIM: This tag sets the time step in fs for the molecular dynamics run.
  • NSW: This tag sets the number of steps performed in the molecular dynamics run.
  • TEBEG: If a thermostat is used define the desired temperature at which the molecular dynamics calculations should run.
  • ISIF (optional).
  • MDALGO: This tag decides with which thermostat the molecular dynamics calculation is executed. For regular molecular dynamics calculations the thermostat is selected by a one digit number (e.g. 1 for Andersen, 2 for Nose-Hoover etc.). For biased molecular dynamics, metadynamics etc. the thermostat is selected the same way from VASP 6 or higher. In VASP 5.x it is selected by a two digit number where the first digit corresponds to the thermostat analogously to regular molecular dynamics and the second digit corresponds to the molecular dynamics type (e.g. 11 metadynamics with Andersen thermostat, 21 metadynamics with Nose-Hoover thermostat etc.). The NVE ensemble is a special case. It is available by selecting the Andersen thermostat and setting no collisions with the heat bath (ANDERSEN_PROB=0).
  • ISIF: In molecular dynamics calculations this tag is used to choose the NVT ensemble or NpT ensemble (the NVE ensemble is a special case!). For ISIF=2 the volume is kept constant and the NVT ensemble is used. Using this tag the stress tensor is calculated and hence the pressure can be monitored. For ISIF=3 the stress tensor (pressure) is kept constant and the NpT ensemble is used. Using this tag the volume is calculated and can be monitored.
• Decide which ensemble to use:
  • NVT ensemble: Set ISIF=2.
  • NpT ensemble: Set ISIF=3.
  • NVE ensemble: Set MDALGO=1 and ANDERSEN_PROB=0.0.
• Decide which thermostat to use (the combination of thermostats and ensembles is given in table):
  • Andersen thermostat: Set MDALGO=1. Also set ANDERSEN_PROB>0.0 to control the stochastic update frequency of the thermostat.
  • Nose-Hoover thermostat: Set MDALGO=2. Also set SMASS>0.0 to control the coupling to the heat bath.
  • Langevin thermostat: Set MDALGO=3. Also set LANGEVIN_GAMMA>0.0 to control the friction parameter. If the NpT ensemble is used (by setting ISIF=3) additionally the friction coefficient of the lattice LANGEVIN_GAMMA_L has to be provided too.
  • Nose-Hoover-chain thermostat: Set MDALGO=4. Also set the NHC_NCHAINS>0 to adjust the length of the Nose-Hoover chain. With NHC_PERIOD the period (coupling strength to the heat bath) of the Nose-Hoover chains thermostat is set.

The following combinations of thermostats and barostats is possible:

	Thermostat
Ensemble	Andersen	Nose-Hoover	Langevin	NHC	CSVR	Multiple Andersen
Microcanonical (NVE)	MDALGO=1, ANDERSEN_PROB=0.0
Canonical (NVT)	MDALGO=1	MDALGO=2	MDALGO=3	MDALGO=4	MDALGO=5	MDALGO=13
	ISIF=2	ISIF=2	ISIF=2	ISIF=2	ISIF=2	ISIF=2
Isobaric-isothermal (NpT)	not available	not available	MDALGO=3	not available	not available	not available
			ISIF=3
Isoenthalpic-isobaric (NpH)	MDALGO=3, ISIF=3, LANGEVIN_GAMMA=LANGEVIN_GAMMA_L=0.0

Initial Velocities

In VASP, velocities are initialized randomly following the Maxwell-Boltzmann distribution, adjusted to the initial temperature set by TEBEG in the INCAR file. Alternatively, starting velocities can be provided in the POSCAR file, either manually or from a previous VASP run. Manually entered velocities must follow the format and units specified on the POSCAR page.

Compilation

To run molecular dynamics calculation VASP has to be compiled using the -Dtbdyn precompiler flag in the makefile.include file. A sample input using this tag would look like this:

CPP     = $(CPP_) -DHOST=\"IFC9_fftw\" \
          -Dkind8 -DNGXhalf  -DCACHE_SIZE=12000 -DPGF90 -Davoidalloc  \
          -Dtbdyn

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