INCAR: Difference between revisions
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*Bare metal slabs representing closed surfaces. | *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 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]. | 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: [http://dx.doi.org/10.1016/0009-2614(80)80396-4] [http://cms.mpi.univie.ac.at/vasp/vasp/IBRION_1_I.html]. | ||
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 ~0.25. The algorithm is stable even for weird, high-stressed structures | 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 ~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)==== | ====Conjugated Gradient algorithm (IBRION=2)==== | ||
Is the recommended algorithm of 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 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)==== | ====Damped MD and QUICKMIN (IBRION=3)==== | ||
If IBRION=3 is selected, VASP will use the QUICKMIN, unless SMASS value is feet in INCAR. | |||
====Transition state optimization (IBRION=44)==== | ====Transition state optimization (IBRION=44)==== | ||
Revision as of 16:02, 23 October 2012
By controling some parameters in the INCAR file, you can greatly increase the efficiency of your calculations.
Back to Núria López and Group page
Basic parameters
Ionic movement parameters
You can find more information about this topic in the VASP manual [1]
Ionic relaxation
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 ~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, unless SMASS value is feet in INCAR.
Transition state optimization (IBRION=44)
Others ionic updates
Molecular Dynamics (MD) (IBRION=0)
See Molecular Dynamics with VASP