Elastic band method

If the elastic band method is used on the T3D scaLAPACK has to be switched of (see 6.54).

VASP.4.X supports the elastic band method to calculate energy barriers. The INCAR, KPOINTS, and POTCAR files must be located in the directory in which VASP is started. In addition, a set of subdirectories (numbered 00,01,02...) must be created, and each subdirectory must contain one POSCAR file. The tag

  IMAGES= number of images

(specified in the INCAR file) forces VASP to run the elastic band method. The number of nodes must be dividable by the number of images (the NPAR switch can still be used as described above). VASP divides the nodes in groups, and each group then works on one ``image''. The first group of nodes reads 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.

All output (OUTCAR, WAVECAR, CHGCAR etc.) is 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. The usual stdout of the images 02,03,...,number of images is redirected to the files 02/stdout, 03/stdout etc. (only image 01 writes to the usual stdout). In addition to the IMAGES tag, a spring constant can be supplied in the SPRING tag. The default is

 SPRING=-5

For SPRING$=$0, each image is only allowed to move into the direction perpendicular to the current hyper-tangent, which is calculated as the normal vector between two neighboring images. This algorithm keeps the distance between the images constant to first order. It is therefore possible to start with a dense image spacing around the saddle point to obtain a finer resolution around this point.

The nudged elastic band method[59,60] 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 quite 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 we 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,

$$\sum_{i=1,N_{ions}} \vec R_i^{\alpha}$$ (14)

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 appropriated, this can be done with a small script called

  interpolatePOS

found in  vamp/scripts/. 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.