INCAR

Back to Núria López and Group page. By controling some parameters in the INCAR file, you can greatly increase the efficiency of your calculations.

Ionic movement parameters

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

Ionic relaxation

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]

Electronic relaxation parameters

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