Editing INCAR (section)

=Ionic steps=

==Static (single-point) calculations==
For a static calculation (e.g. No ions update), set:
 ionic steps:         
   IBRION =  -1     #  
   NSW    =   1     # 
NSW values larger than 1 can be use to converge particularly difficult electronic structures without updating the ions.  

==Ionic relaxations==

===DIIS algorithm (IBRION=1)===
The DIIS algorithm converges fast in systems that:
*Are close to an energy minimum (or maximum).
*Have low degrees of freedom.
Examples of those systems are:
*Molecules in vacuum with short backbones (tert-butanol or shorter).
*Bare metal slabs representing closed surfaces. 
*Pre-converged transition states. 

For relaxations use: 
 ionic steps:         
   IBRION =   1     #  
   POTIM  =   0.25  # Between 0.10 and 0.40. 
   EDIFFG =  -0.02  # In eV/Å. 
   NSW    = 100     # Between 50 and 100.      

For '''already pre-converged transition states''' use: 
 ionic steps:         
   IBRION =   1     #  
   POTIM  =   0.01  # Lower than 0.03.                  
   EDIFFG =  -0.05  # In eV/Å
   NSW    =  50     # Between 25 and 50.      

For more information: [http://dx.doi.org/10.1016/0009-2614(80)80396-4] [http://cms.mpi.univie.ac.at/vasp/vasp/IBRION_1_I.html].

Back to [[Núria López and Group]] / [[Scripts_for_VASP]].

===CG algorithm (IBRION=2)===

Is the recommended algorithm if you don't know what to do (See Ionic Relaxation Methods in [http://www.vasp.at/index.php?option=com_content&view=article&id=49&Itemid=57]). It is faster and more stable than DIIS for medium and large systems, and always converges into a minimum. 

 ionic steps:         
   IBRION =   2     #  
   POTIM  =   0.150 # Between 0.15 and 0.25.                  
   EDIFFG =  -0.020 # In eV/Å
   NSW    = 100     #  

===Damped MD and QUICKMIN (IBRION=3)===

Recomended to use along NEBs for big systems or in conjunction with NEBs.
 ionic steps:         
   IBRION =   3     #  
   POTIM  =   0.011 # Always lower than 0.020.                   
   EDIFFG =  -0.020 # In eV/Å
   NSW    = 100     #  

POTIM should be very small, lower than the reciprocal of the highest eigenvalue in the Hessian Matrix. For hydrogen-containing systems, this value is around 0.025. Values lower than 0.017 are usually very safe. 
 
Back to [[Núria López and Group]] / [[Scripts_for_VASP]].

==Frequencies (IBRION=5,6)==
See [http://cms.mpi.univie.ac.at/vasp/vasp/IBRION_5_IBRION_6.html]. After calculating frequencies, you can visualize them with [[Jmol]]. 

 ionic steps: 
   IBRION =   5     #  
   POTIM  =   0.02  # Always lower than 0.020.                   
   NFREE  =   2     # 

Tip: If your system was obtained with a tight convergence criteria (eg: EDIFFG=-0.02), you can use NFREE=1 instead of 2. You will have a reasonable accuracy in the frequencies with half the computational cost.

Back to [[Núria López and Group]] / [[Scripts_for_VASP]].

==Finding transition states== 

A robust search for transition states starts by preconverging using a [[NEB]]. To run a NEB, please make sure you are using a VTST-modified version of VASP and put these flags on your INCAR file: 
 ionic steps:         
   IMAGES =   4     # 
   LCLIMB =   F     # 
   IBRION =   3     # 
   POTIM  =   0.011 # 
   SPRING =  -5     # 
   NSW    = 100     # Use less than 200 steps for NEBs

IBRION=3 works very well for NEBs. Do never use IBRION=2. If you have more than one local maximum, modify your initial and final states and repeat the NEB. If the initial and final states are too different, you can also do a zoom or increase the number of images. Avoid using more than 6 images. You can [[INCAR#Tips_for_efficient_electronic_relaxations|tweak EDIFF and NELMIN]] in the ''electronic steps'' section to make it run faster.    

Once the NEB is pre-converged, launch a CI-NEB: 
 ionic steps:         
   IMAGES =   4     # 
   LCLIMB =   T     # 
   IBRION =   1     # 
   POTIM  =   0.150 # 
   SPRING =  -5     # 
   NSW    =  50     # Use less than 100 steps for CI-NEB 

After 50 steps, you should have a high-quality transition state localized as the image with highest energy. CI-NEB works best with IBRION=1.  

Finally, do a [[INCAR#Frequencies_.28IBRION.3D5.2C6.29|frequencies calculation]] to check that all frequencies are real except by one that represents the transition state. 

The transition staten can be further refined by applying [[INCAR#DIIS_algorithm_.28IBRION.3D1.29|IBRION=1]] with a very small POTIM and by using the [[IDM|Improved Dimer Method]]. 

Back to [[Núria López and Group]] / [[Scripts_for_VASP]].

==Ab-Initio Molecular Dynamics==
VASP manual: [http://cms.mpi.univie.ac.at/vasp/vasp/IBRION_0.html]

To avoid unphysical variations in the temperature of your BOMD, start with a pre-heating from 0K to your working temperature. Also break the symmetry, which is particularly important if your initial structure is highly symmetric. The following text replaces the '''ionic steps''' section in your INCAR file:   

 # molecular dynamics
   IBRION =   0
   POTIM  =   1.00  # timestep in fs. 
   NSW    = 100     # 3700*3.00 = 11000 fs = 11.1 ps  
   SMASS  =   0     # thermal bath NVT 
   TEBEG  =   0     # initial temperature
   TEEND  = 300     # final temperature 
   ISYM   =   0     # symmetry off, here it is compulsory

Then, for the rest of the runs keep constant the thermal bath. For BOMD you can do long runs. Be careful that they finish before the time-limit is reached, for instance in MareNostrum4: 
 # molecular dynamics
   IBRION =   0
   POTIM  =   1.00  # timestep in fs. 
   NSW    =1000     # 
   SMASS  =   0     # thermal bath NVT 
   TEBEG  = 300     # initial temperature
   TEEND  = 300     # final temperature 
   ISYM   =   0     # symmetry off, here it is optional

Back to [[Núria López and Group]] / [[Scripts_for_VASP]].