INCAR

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By controling some parameters in the INCAR file, you can greatly increase the efficiency of your calculations.

Basic parameters

ISTART

ICHARG

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Ionic movement parameters

You can find more information about this topic in the VASP manual [1]

Ionic relaxation

The ionic relaxation loop is performed

There are five ionic relaxation algorithms implemented in VASP 5.x:

RMM-DIIS algorithm (IBRION=1)

The RMM-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 (e.g. tert-butanol is one of the largest).
• Bare metal slabs representing closed surfaces.

The algorithm builds up an approximation of the Hessian matrix, taking the last NFREE steeps. The forces should be calculated precisely, therefore you'd better set NELM=4 or even NELM=8. This forces a minimum of 4 to 8 electronic steps between each ionic step, and guarantees that the forces are well converged at each step. The NFREE number should be lower than the number of degrees of freedom, and it is set by VASP considering several constrains, unless it is specified by the user. For large values of NFREE this algorithm may diverge. For more information: [2] [3].

A test made with a gas-phase molecule in different conditions established that good POTIM values are between 0.15-0.40, with an optimum of POTIM=0.25. The algorithm is stable up to POTIM=0.80, even for weird, high-stressed structures. POTIM values smaller than 0.15, or larger than 0.40, only decrease the speed of convergence.

Conjugated Gradient algorithm (IBRION=2)

Is the recommended algorithm of you don't know what to do (See Ionic Relaxation Methods in [4]). It is faster and more stable than RMM-DIIS for medium and large systems, and always converges into a minimum (?).

The CG algorithm less sensitive to POTIM, and is stable for both stressed and pre-converged structures up to POTIM=1.00 (No larger values tested). However, the optimal value for both conditions is POTIM=0.15~0.20. Values lower than 0.15 reduces the speed of convergence.

Damped MD and QUICKMIN (IBRION=3)

If IBRION=3 is selected, VASP will use the QUICKMIN algorithm by default. If a SMASS value is feet in INCAR, VASP will switch to the damped Molecular Dynamic algorithm. Both suppose to be more efficient than CG and RMM-DIIS in large systems, but dMD can be faster than QUICKMIN if a good set of SMASS and POTIM is selected.

A good set of values for adsorption systems (tested on methanol on Pt(111) 2×2 surface) with reasonable initial guests is SMASS=0.20 and POTIM=0.16 ~ 0.20. The optimal set of values may change according to the size and nature of the system, therefore, further test are needed for:

• QUICKMIN algorithm.
• dMD algorithm, with POTIM values larger than 0.20
• Systems that are larger or different in chemical nature.

For IBRION=1,2 and 3 (dMD), POTIM is defined as a (dimensionless?) scaling constant for the forces. For QUICKMIN it has units (s/kg).

A good selection of SMASS can improve the convergence speed by around one order of magnitude after 50 ionic steeps.

Transition state optimization (IBRION=44)

Dimer method of G. Henkelman and H. Jónsson (J.Chem.Phys.,111,7010(1999)), implemented by Heyden et al[5]. Further test are needed in our group.

Others ionic updates

Molecular Dynamics (MD) (IBRION=0)

See Molecular Dynamics with VASP VASP manual: [6]

Thermodynamics (IBRION=5,6)

See [7]

Tip: If your system was obtained with a tight convergence criteria (eg: EDIFFG=-0.01), you can use NFREE=1 instead of 2 and reach a reasonable accuracy in the frequencies calculation with half of the computational cost.

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Electronic relaxation parameters

For pre-converge a calculation, set:

EDIFF  = 1E-3 ! Or 1E-4 for each moving nucleus in your POSCAR file
ENCUT  = 250  ! Or the higher ENMIN value in your POTCAR file
NSW    = 4    ! Or 5. Increase this value too much may rise computational adding precision to the forces.  

For converge a calculation, set:

EDIFF  = 1E-5 ! 
ENCUT  = 450  ! This value must be consistent with all your converged calculations.
NSW    = 3    ! Or 2, that is the default.

What to do when experiencing convergence problems

If you have problems to reach convergence in the first electronic loop, and you are not reading WAVECAR, set:

NELMIN = 15   ! Number of non self-consistent electronic steps at the beginning (w/o CHG update)

Otherwise, vary those mixing scheme parameters (you can play with them):

AMIX   = 0.1
BMIX   = 0.01

If problems persist, increase BMIX:

BMIX   = 3.0
AMIN   = 0.01  ! Do not confuse with AMIX

If problems persist, read this [[8]]

For further reading: [[9]]

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Aditional parameters

van der Waals contributions

VdW_forces

Files to write

There are several flags to state what files to be written.

 FLAG      FILE          DEFAULT
 LWAVE  => WAVECAR       .TRUE.  [10]
 LCHARG => CHG / CHGCAR  .TRUE.  [11] [12]
 LVTOT  => LOCPOT        .FALSE. [13]  
 LELF   => ELFCAR        .FALSE. [14]  
 PARCHG => PARCHG        .FALSE. 

Tip: Set LWAVE=.FALSE. and LCHARG=.FALSE. to avoid that VASP writes the WAVECAR, CHG & CHGCAR files. Set those values to .TRUE. if you plan to start a new calculation based on those results.

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