Molecular-dynamics calculations

Molecular dynamics (MD) is used to study the movement of atoms over time in a given thermodynamic ensemble. This makes MD useful whenever a static ground-state calculation is insufficient, e.g., to study thermal disorder, diffusion, structural fluctuations, phase stability, melting, or how a system equilibrates under realistic conditions.

In an ab initio MD, the forces along the trajectory can be computed directly from DFT, or they can be provided by a machine-learned force field (MLFF) once a reliable model is available. In practice, MD in VASP can therefore be used for both accurate short first-principles trajectories and much longer simulations involving more atoms based on native MLFF workflows or external models, such as GRACE. This page explains how to set up and validate standard MD calculations in VASP, as well as how to avoid the most common numerical and physical pitfalls. More advanced sampling methods are covered under "advanced molecular-dynamics sampling".

Step-by-step instructions

Step 1: Prepare a stable starting structure

Generate a POSCAR containing a large enough cell. In practice, MD usually requires a substantial number of ions so that the trajectory samples a meaningful distribution of local environments. If the cell is too small, the statistics will be poor, and the atoms, defects, or local distortions may interact too strongly with their periodic images.

You may want to consider proceeding with a structure optimization. If the initial structure is still strained or carries large residual forces, the MD run will spend its initial steps removing artificial stress rather than of sampling the desired physical motion. This often results in to unstable trajectories, poor temperature control, and, in MLFF training workflows, low-quality training data. Alternatively, you can use a CONTCAR file from a previous MD run as a starting point to continue the trajectory, because in that case it already contains ionic velocities in addition to the structure.

If you plan to run ab initio MD, relax the structure with the same ab initio setup that you will later use for the trajectory.The same consistency is needed for MLFF-based MD: either relax with the same MLFF that will drive the dynamics, or generate the reference structures with the same ab initio settings used for training. Disable symmetry with ISYM = 0 for MD, since thermal motion generaly breaks the symmetry of the relaxed structure.

Mind: Before starting the MD, do not forget to copy CONTCAR to POSCAR.

cp CONTCAR POSCAR

If POSCAR does not contain velocities, VASP will randomly generate them at the beginning of the run. While this is fine for equilibration, it makes it harder to compare trajectories from separate runs directly.

Step 2: Set the core MD tags

Activate MD with IBRION = 0, then define the trajectory length, timestep, temperature control, and cell constraints.

The key MD tags are:

NSW sets the number of ionic steps and therefore the total simulation length.
POTIM sets the time step in fs and must be supplied for MD runs. It must be small enough to resolve the fastest ionic motion.
TEBEG and TEEND define the initial and target temperatures for thermostat-based runs.
MDALGO selects the thermostat and integration scheme. More on that in the next step.
ISIF determines whether the cell can change shape and volume or is fixed.

Choose POTIM conservatively. Systems containing hydrogen bonds or stiff bonds usually require smaller timesteps than heavy, weakly bound systems. Reduce POTIM and retest if the total energy drifts strongly in NVE, if the atoms show unrealistically large displacements, or if the electronic self-consistent field (SCF) cycle becomes erratic.

For long trajectories, also decide how often data should be written. NBLOCK controls how often ionic configurations are written to XDATCAR and how often pair-correlation and DOS-related quantities are accumulated. In MLFF prediction runs, use ML_OUTBLOCK to more broadly reduce screen and file output frequency.

Tip: The atomic masses can be intentionally changed for MD, by setting the POMASS tag in the INCAR. By default the masses are read from POTCAR. This can be useful for increasing POTIM for light atoms such as hydrogen, but it changes the physical dynamics and should be avoided when mass-dependent observables are the target.

Step 3: Select the ensemble and cell dynamics

Select an ensemble based on the physical conditions you want to sample. Because VASP determines the ensemble by combining MDALGO and ISIF, your choice of thermostat directly impacts your available cell degrees of freedom. For instance, while the Langevin thermostat (MDALGO = 3) flexibly supports both NVT and NpT simulations, other algorithms do not allow independent configuration of the cell.

Canonical ensemble (NVT): use this to run simulations at a fixed number of particles (N), fixed volume (V) and constant temperature (T). Several thermostats can be used here, including Andersen (MDALGO = 1), Nosé–Hoover (MDALGO = 2), Langevin (MDALGO = 3), Nosé–Hoover chain (MDALGO = 4), CSVR (MDALGO = 5), and multiple Andersen thermostats (MDALGO = 13). Keep the cell fixed with ISIF < 3; ISIF = 2 is a common choice because it also reports the full stress tensor.
Micro–canonical ensemble (NVE): use this only after equilibration. This ensemble is very useful because the atoms will be propagated by the MLFF or DFT forces only. So there will be no artificial thermostat data added to the velocities. This ensemble might be helpful if there is interest in self correlation functions. For example velocity auto-correlation functions might be of interest because phonon DOS can be obtained from these. It is treated as a special case where a thermostat is selected but effectively switched off. The simplest way to do this is with MDALGO = 1 and ANDERSEN_PROB = 0.0. Another option is MDALGO = 2 with SMASS = -3, which disables the Nosé–Hoover thermostat and yields NVE dynamics. Keep the cell fixed with ISIF < 3. Note here that the choice of the thermostat will determine the propagation scheme which is used for the NVE simulation.
Isothermal–isobaric ensemble (NpT): use this when pressure and volume fluctuations are part of the problem. This will be the case when for example phase transitions should be studied under which the simulation box will change. In VASP, this is implemented for Langevin dynamics, i.e., MDALGO = 3 together with ISIF = 3. The Langevin thermostat requires the following additional tags: LANGEVIN_GAMMA for the ions and LANGEVIN_GAMMA_L for the lattice. PMASS controls the fictitious lattice mass.
Isoenthalpic–isobaric ensemble (NpH): use this when you want constant pressure without a thermostat hitting at you system. This ensemble is interesting when studying for example crystallization processes. Crystallization converts potential energy into kinetic energy. An (NpT) thermostat artificially drains this kinetic energy to keep temperature flat, killing the natural heating that regulates the real nucleation rate. Again, one has to use the Langevin route with MDALGO = 3 and ISIF = 3, but with LANGEVIN_GAMMA = 0 and LANGEVIN_GAMMA_L = 0, which switches off both ionic and lattice thermostatting.

For most workflows, begin with NVT and then validate the timestep and force quality in NVE. Only use NpT when the cell should fluctuate. NpT can lead to irreversible cell deformations for liquids or systems with limited long-range order unless lattice constraints are applied.

	Thermostat
Ensemble	Andersen	Nosé-Hoover	Langevin	Nosé-Hoover chain	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
Canonical (NVT)	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
Isobaric-isothermal (NpT)	not available	not available	ISIF=3	not available	not available	not available
Isoenthalpic-isobaric (NpH)	MDALGO=3, ISIF=3, LANGEVIN_GAMMA=LANGEVIN_GAMMA_L=0.0

A minimal fixed-cell NVT starting point is:

IBRION = 0
NSW    = 5000
POTIM  = 1.0
TEBEG  = 300
TEEND  = 300
MDALGO = 2
SMASS  = 0
ISIF   = 2
ISYM   = 0

For variable-cell NpT with Langevin dynamics, start from:

IBRION           = 0
NSW              = 5000
POTIM            = 1.0
TEBEG            = 300
TEEND            = 300
MDALGO           = 3
ISIF             = 3
LANGEVIN_GAMMA   = 10.0 10.0 # For two species
LANGEVIN_GAMMA_L = 10.0
PMASS            = 1000
ISYM             = 0

These are just starting points for MD control; add the converged electronic and force-provider settings from the next step.

For the Langevin parameters, begin with moderate friction and only adjust if a clear problem appears in the short test run. If LANGEVIN_GAMMA or LANGEVIN_GAMMA_L is too small, temperature or pressure equilibration may be very slow. If they are too large, the dynamics become overdamped, and time-dependent observables become less meaningful. PMASS controls how quickly the cell responds. Too small values can lead to violent volume fluctuations, while too large values makes the cell relaxation very sluggishly. For liquids or soft systems, closely monitor the cell evolution and use the ICONST file to constrain the lattice if the box begins to deform irreversibly.

Step 4: Choose how the forces are computed

Select the method for force generation, as this choice dictates the computational expense and determines whether the workflow incorporates machine learning training.

Ab initio MD: all forces are computed from DFT at every step. This is the standard setup and typically the best starting point for a new system.
Native VASP MLFF, on-the-fly: enable ML_LMLFF = .TRUE. to activate machine-learned force fields. With ML_MODE = train, VASP performs on-the-fly learning. MLFF predictions are used when the estimated error is low enough. Additional ab initio calculations are triggered when new reference data are needed. Therefore, this mode still requires a complete ab initio setup, including INCAR, KPOINTS, and POTCAR. For the full workflow, see: "Machine learning force field calculations: Basics".
Native VASP MLFF, prediction-only: after training and refitting the force field, use ML_MODE = run to perform MD with MLFF predictions only. This mode does not generate new ab initio data, so it should only be used once the applicability of the force field has been verified.
GRACE in VASP: GRACE is an external pretrained force field and is also a prediction-only method. Use ML_LMLFF = .TRUE., ML_MODE = run, and ML_TYPE = grace with a compatible build and an available GRACE model.

In practice, the practical distinction is simple: ab initio MD computes every step directly from DFT; on-the-fly MLFF combines MD with automatic training; and both VASP-native ML_MODE = run and GRACE are prediction-only modes.

An ab initio MD run also requires electronic settings that maintain a stable SCF cycle throughout the trajectory. A conservative starting point is:

PREC  = Accurate
EDIFF = 1E-6
ALGO  = Normal

Combine these with the converged ENCUT, smearing choice, and k-point mesh from the preceding relaxation. For on-the-fly MLFF runs, the same ab initio settings must be present because VASP triggers reference electronic calculations whenever new training data are needed.

Mind: Keep the exchange-correlation functional, PAW datasets, cutoff, and k-point strategy consistent between relaxation, MLFF training, and MD. Changing these settings changes the underlying potential-energy surface, meaning the relaxed structure and forces are no longer fully compatible.

To convert the previous MD input into an on-the-fly MLFF run, add:

ML_LMLFF = .TRUE.
ML_MODE  = train

For prediction-only MLFF, use:

ML_LMLFF = .TRUE.
ML_MODE  = run

For GRACE, add ML_TYPE = grace to the prediction-only setup.

Mind: If the cell is allowed to change with ISIF = 3, monitor the cell shape and volume carefully. The PAW basis is fixed at the beginning of the calculation, so significant cell changes can lead to Pulay-stress errors and unreliable pressure readings.

Step 5: Test the run before extending it

Begin with a short trajectory and examine the temperature, forces, stress, and electronic convergence before committing to a long run. Ensure that the thermostat drives the system toward the intended temperature, that no atom leaves the physically meaningful structure, and that the SCF cycle remains stable along the trajectory.

In practice, check OSZICAR for the temperature and total energy evolution, OUTCAR for force maxima, stress, and SCF convergence behavior, and XDATCAR or CONTCAR for obviously unphysical atomic motion. Warning signs include repeated electronic nonconvergence, large force spikes, a steady temperature drift away from the target in NVT, strong total energy drift in NVE, and rapidly growing cell distortions in variable-cell runs. If any of these occur, first reduce POTIM. Then, tighten the electronic settings. If needed, fall back to a simpler fixed-cell NVT test.

If the short test behaves sensibly, continue from CONTCAR with the same physical and electronic settings. To restart, copy CONTCAR to POSCAR. If the CONTCAR file was written by an MD run it may also contain the ionic velocities. This allows the trajectory to continue smoothly instead of starting with newly randomized velocities. Use a short NVE segment after equilibration to assess whether the timestep and force convergence are adequate.

Mind: Equilibration and production serve different purposes. Use the initial part of the trajectory to allow the temperature, pressure, volume, and local structure to relax away from the initial configuration. Only after these quantities have fluctuated around a steady state should the trajectory be treated as production data for calculating averages, diffusion coefficients, performing structural analyses, and determining other observables.

Recommendations and advice

Use a generous cutoff for variable-cell runs. With ISIF = 3, the PAW basis does not update during the run. Therefore, large cell distortions can increase Pulay-stress errors and degrade the pressure.
For long trajectories, increase NBLOCK if XDATCAR and related MD output become unnecessarily large. For MLFF prediction-only runs, use ML_OUTBLOCK for output throttling. It also sets a lower bound for NBLOCK.
Restart from CONTCAR and retain the velocities when continuing a trajectory.
Do not interpret a very short trajectory as thermodynamic sampling. Equilibration and production are separate parts of an MD workflow.
Most thermostat and barostat options selected through MDALGO require a build compiled with -Dtbdyn.

Common pitfalls:

Starting from an highly strained structure, which often causes large forces and unstable dynamics.
Using a supercell that is too small or containing too few ions results in poor statistics and enhances finite-size artifacts from periodic images.
Choosing a timestep that is too large for the fastest vibrational modes.
Allowing poor electronic convergence, which directly corrupts the forces.
Changing the cutoff, smearing, or the k-point mesh between related MD or MLFF runs.
Using variable-cell dynamics without ensuring that the pressure and Pulay stress are under control.
Treating thermostat-controlled trajectories as if they automatically provide reliable dynamical observables is also a mistake.
Restarting from CONTCAR without confirming that velocities are present and physically consistent.
Using MLFF or GRACE production runs outside the range of structures represented in the training data.