Restrictions in VASP.4.X and restrictions due to parallelization

In most respects VASP.4.X should behave like VASP.3.2. However in VASP.4.4, IALGO=48 was redesigned to work more reliable in problematic cases. Therefore the iteration history might not be directly comparable. VASP.4.X also subtracts the atomic energies in each iteration, VASP.3.2 does not. Once again this means that the energies written in each electronic step are not comparable.

The parallel version (i.e. if VASP is compiled with the MPI flag) has some further restriction, some of them might be removed in the future:

Here is a list of features not supported by VASP.4.4 running on a parallel machine:

• VASP.4.4 ( VASP.4.5 does not posses this restriction): The most severe restriction is that it is not possible to change the cutoff or the cell size/shape on restart from existing WAVECAR file. This means that if the cell size/shape and or the cutoff has been changed the WAVECAR should be removed before starting the next calculation (actually VASP will realize if the cutoff or the cell shape have been changed and will proceed automatically as if the WAVECAR file does not exist). The reason for this restriction is that the re-padding (i.e. the redistribution of the plane wave coefficients on changing the cutoff sphere) would require a sophisticated redistribution of data and the required communication routines are not implemented at present.

  As a matter of fact, it is of course possible to restart with an existing WAVECAR file even if the number of nodes has changed. The only point that requires attention is that changing the NPAR parameter might also effect the number of bands (NBANDS). WAVECAR files can only be read if the numbers of bands is strictly the same on the file and for the present run. In some cases, it might be required to set the number of bands explicitly in the INCAR file by specifying the NBANDS parameter.

• Symmetry is fully supported by the parallel version, BUT we have used a brute force method to implement it. The charge density is first merged from all nodes, then symmetrized locally and finally the result is redistributed onto the nodes. This means that the symmetrization of the charge density will be very slow, this can have serious impact on the total performance.

  In VASP.4.4.3 (and newer version) this problem can be reduced by specifying ISYM=2 instead of ISYM=1. In this case only the soft charge density and the augmentation occupancies are symmetrized, which results in precisely the same result as ISYM=1 but requires less memory. ISYM=2 is the default for the PAW method.

• Partial local DOS is only supported with parallelization over plane wave coefficients but not with parallelization over bands. The reason is that some files (like PROCAR) have a rather complicated band-by-band layout, and it would be complicated to mimic this layout with a data distribution over bands.