IBRION
IBRION = 1  0  1  2  3  5  6  7  8  11  12  40  44
Default: IBRION  = 1  for NSW=−1 or 0 
= 0  else 
Description: determines how the crystal structure changes during the calculation:
Molecular dynamics
In moleculardynamics (MD) simulations the positions of the ions are updated using a classical equation of motion for the ions. There are several algorithms for the time propagation in MD controlled by selecting MDALGO and the choice of the thermostats. The MD run performs NSW timesteps of length POTIM.
Frequently, performing an abinitio calculations in every step of an MD simulation is too expensive so that machinelearned force fields are needed.
Tip: In order to limit the output of the MD simulation, control the verbosity by setting NWRITE=0,1, or reduce the frequency of output using ML_OUTBLOCK, NBLOCK, or KBLOCK. 
Structure optimization
VASP optimizes the structure based on the degrees of freedom selected with the ISIF tag and (if used) the selective dynamics POSCAR file. Generally, the larger the number of degrees of freedom, the harder it is to find the optimal solution. To find the solution, VASP provides multiple algorithms:
 RMMDIIS (IBRION=1) reduces the forces by linear combination of previous positions. It is the method of choice for larger systems (>20 degrees of freedom) that are reasonably close to the groundstate structure.
 Conjugate gradient (IBRION=2) finds the optimal step size along a search direction. It is a robust default choice but may need more iterations than RMMDIIS.
 Damped molecular dynamics (IBRION=3) runs a MD simulation with decreasing velocity of the ions. Use this for large systems far away from the minimum to get to a better starting point for the other algorithms.
Consult the structure optimization page for advise on how to choose the optimization algorithm.
Computing the phonon modes
The secondorder derivatives of the total energy with respect to ionic positions of ion in the direction , is computed using a firstorder derivative of the forces . Then, the dynamical matrix is constructed, diagonalized, and the phonon modes and frequencies of the system are reported in the OUTCAR file and vaspout.h5. Also see theory on phonons.
Tip: It may be necessary to set EDIFF because the default (EDIFF = 1E4 ) often results in unacceptably large errors.

VASP implements two different methods to compute the phonon modes and can use symmetry to reduce the number of computed displacements:
IBRION = 5
finite differences without symmetryIBRION = 6
finite differences with symmetryIBRION = 7
densityfunctionalperturbation theory without symmetryIBRION = 8
densityfunctionalperturbation theory with symmetry
For finite differences, the elastic tensors and internal strain tensors is computed for ISIF>=3.
Compute Borneffective charges, piezoelectric constants, and the ionic contribution to the dielectric tensor by specifying LEPSILON = .TRUE.
(linear response theory) or LCALCEPS = .TRUE.
(finite external field).
Also see computing the phonon dispersion and DOS.
Analyzing transition states
To study the kinetics of chemical reactions, one may want to construct transition states or follow the reaction path. For the analysis of transition states the following methods are available:
 Setting
IBRION = 40
, you can start from a transition state and monitor the energy along an intrinsicreaction coordinate (IRC). The IRC calculations section describes this method.  With the improved dimer method (
IBRION = 44
), you can search for a the transition state starting from an arbitrary structure in the investigated phase space.  The nudged elastic bands method finds an approximate reaction path based on the initial and final structure, i.e., reactant and product.
Interactively supplied positions and lattice vectors
Occasionally, you may want to run VASP for related structures where the overhead of restarting VASP is significant. In these scenarios, VASP provides the following alternatives
 With
IBRION = 11
, you can provide new structures via the standard input. For ISIF>=3, a complete POSCAR file is read, otherwise just the positions in fractional coordinates.
Related tags and articles
Related tags: NSW, POTIM, MDALGO, SMASS, NFREE, ISIF, LEPSILON, LCALCEPS
Related files: POSCAR, CONTCAR, XDATCAR, vaspout.h5
Related topics and howto pages: Timepropagation algorithms in molecular dynamics, Structure optimization, Selective dynamics, Computing the phonon dispersion and DOS, Transition states, IRC calculations, Improved Dimer Method, Writing a Python plugin