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Molecular -- Dynamics

About which files do you have to worry:

INCAR
POSCAR
POTCAR
KPOINTS
For a molecular dynamics, we recommend the use of PREC=Normal, although PREC=Low yields often satisfactory results. Here an example INCAR file:
SYSTEM =  Si
# electronic degrees                                                                                                   
ENCUT = 120                                 
LREAL = A                      # real space projection                         
PREC  = Normal                 # chose Low only after tests
EDIFF = 1E-5                   # do not use default (too large drift)          
ISMEAR = -1 ; SIGMA = 0.172    # Fermi smearing: 2000 K 0.086 10-3
ALGO = Very Fast               # recommended for MD (fall back ALGO = Fast)     
MAXMIX = 40                    # reuse mixer from one MD step to next          
NCORE= 4 or 8                  # one orbital on 4 cores                        
ISYM = 0                       # no symmetry                                                 
NELMIN = 4                     # minimum 4 steps per time step, avoid breaking after 2 steps
                
# MD (do little writing to save disc space)                                    
IBRION = 0 ; NSW =     100 ;  NWRITE = 0 ; LCHARG = .FALSE. ; LWAVE = .FALSE.  
TEBEG =   2000 ; TEEND =  2000                                                 
# canonic (Nose) MD with XDATCAR updated every 50 steps                        
SMASS = 3 ;  NBLOCK = 50 ; POTIM = 1.5                                         
# micro canonical MD with temperature scaling every 50 steps
# good for equlibration but usually better to use Nose thermostat
#SMASS = -1 ; NBLOCK = 50 ; POTIM = 1.5
Use ALGO=Very Fast (RMM-DIIS for electrons) for large molecular dynamics runs. One should also evaluate the projection operators in real space (LREAL=A) to speed up the calculations, and it is recommended to use at least 4 electronic iterations per ionic step (NELMIN = 4). For surface or difficult systems, you might need to increase this value to NELMIN = 8.

Special consideration are required for the parameters BMIX and MAXMIX: it is usually desirable to use optimal mixing parameters for molecular dynamics simulations. This can be done by performing a few static calculations with varying AMIX and BMIX parameters and do determine the one leading to the fastest convergence. However, in the current versions of VASP, the dielectric function is reused when the ions are updated (an optimal AMIX and BMIX is no longer that important). The dielectric function is reused after ionic updates, if MAXMIX is set. MAXMIX should be about three times as larger as the number of iterations required to converge the electronic orbitals in the first iteration.

After performing one MD run, it is possible to continue to run, by copying the CONTCAR to POSCAR file and restarting VASP. Since the CONTCAR file is written after every single step, it is also possible to restart the molecular dynamics from a crashed run.


next up previous contents index
Next: Simulated annealing Up: Examples Previous: Dimers   Contents   Index
N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at