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# IMAGES

IMAGES = [integer]
Default: IMAGES = 0

Description: IMAGES defines how many instances of VASP are started. These instances execute VASP calculations in separate directories 01, 02, 03, etc. This flag is used to support elastic band calculations, parallel tempering, thermodynamic integration as well as other scenaria where a simultaneous calculation of several VASP instances is required.

VASP supports various modes where simultaneous calculations for different INCAR, KPOINTS, POTCAR, or POSCAR file are performed. The elastic band method to calculate energy barriers, as well as parallel tempering are explained in this section.

Generally a primary INCAR file must be located in the root directory. Most of the other files, including KPOINTS, POTCAR, POSCAR can either reside in the subdirectories (named 00, 01, 02...) or in the root directory. The files in the subdirectories take precedence over those in the root directory (for details see the paragraph "File handling" below). The tag

IMAGES = number of images


forces VASP to execute multiple instances, performing calculations in the subdirectors 01, 02, .., number of images. The number of nodes must be dividable by the number of images (the tags NCORE, KPAR or NPAR can still be use and might even be different, if INCAR files are supplied in the subdirectories). VASP divides the nodes in groups, and each group then works on one image 01, 02, ... IMAGES. For nudged elastic band calculations, it is usually sufficient to provide all input files in the root directory, except for the POSCAR files which need to be provided in the subdirectories. The first group of nodes will then read the POSCAR file from the directory 01, the second group from 02 etc. In the elastic band method, the endpoints are kept fixed, and the position of the end points must be supplied in the files 00/POSCAR and XX/POSCAR, where XX is

XX = number of images+1.


(the directories 00 and XX are only required for elastic band calculations).

If the tag LTEMPER=.TRUE. is set in the INCAR file, VASP performs parallel tempering calculations. In this case, it is necessary to supply different INCAR and POSCAR files in each subdirectory. For each subdirectory a different simulation temperature should be supplied using the tags TEBEG in the INCAR files of the subdirectories. For further details, we refer to the description of the tag LTEMPER.

All output files (OUTCAR, WAVECAR, CHGCAR etc.) are all written to the subdirectories. Since no nodes are executing for the positions supplied in the directories 00 and XX, no output files will be created in these sub directories. Image 01 writes to the usual stdout file, located in the directory from which VASP is started, the other images write to the files 02/stdout, 03/stdout etc. In addition to the IMAGES tag, a spring constant can be supplied in the SPRING tag. The default is

SPRING= -5


The nudged elastic band method[1][2] is applied when SPRING is set to a negative value, e.g. SPRING= -5. This is also the recommended setting. Compared to the previous case, additional tangential springs are introduced to keep the images equidistant during the relaxation (remember the constraint is only conserved to first order otherwise). Do not use too large values, because this can slow down convergence. The default value usually works reliably.

One problem of the nudged elastic band method is that the constraint (i.e movements only in the hyper-plane perpendicular to the current tangent) is non linear. Therefore, the CG algorithm usually fails to converge, and we recommended to use the RMM-DIIS algorithm (IBRION=1) or the quick-min algorithm (IBRION=3). Additionally, the non-linear constraint (equidistant images) tends to be violated significantly during the first few steps (it is only enforced to first order). If this problem is encountered, a very low dimensionality parameter (IBRION=1, NFREE=2) should be applied in the first few steps, or a steepest descent minimization without line optimization (IBRION=3, SMASS=2). should be used, to pre-converge the images.

If all degrees of freedom are allowed to relax (isolated molecules, no surface, etc.), make sure that the sum of all positions is the same for each cell. In other words,

${\displaystyle \sum _{i=1,N_{ions}}{\vec {R}}_{i}^{\alpha }}$

must be equal for all images. Otherwise fake forces are introduced, and the images drift against each other (this will not introduce problems during the VASP calculations, but it is awkward to visualize the final results). Often an initial linearly interpolated starting guess is appropriate, this can e.g. be done with a small script called

interpolatePOS


The script also removes as an option the center-of-mass motion.

Finally, we strongly recommend to keep the number of images to an absolute minimum. The fewer images are used, the faster to convergence to the groundstate is. Often, it is advisable to start with a single image between the two endpoints, and to increase the number of images, once this first run has converged.

## References

1. G. Mills, H. Jonsson and G. K. Schenter, Surface Science, 324, 305 (1995).
2. H. Jonsson, G. Mills and K. W. Jacobsen, Nudged Elastic Band Method for Finding Minimum Energy Paths of Transitions, in Classical and Quantum Dynamics in Condensed Phase Simulations, ed. B. J. Berne, G. Ciccotti and D. F. Coker (World Scientific, 1998).