Not enough memory

Nowadays, for standard DFT and hybrid functional calculations, memory is usually not an issues. Furthermore, by increasing the number of cores, the memory requirements per core can be reduced significantly.

The following steps can be taken in order to reduce the memory requirements per core.

• For large many atom systems, increase NCORE to larger values (say to 4, 8 or even the number of cores per node or socket). This allows to decrease the memory requirements per core in particular for the storage of the non-local projectors. Furthermore, real space projection LREAL= A decreases the required memory per core.
• KAR allows to distribute the k-points over cores. Unfortunately, only the calculations are distributed, but the storage of the orbitals is not distributed over cores. This means that using KAR=1 results in the smallest memory requirements per core (but the slowest calculations, since VASP needs to rely on other less efficient parallelization strategies).
• Switch of symmetrisation (ISYM=0). Symmetrisation is done locally on each node requiring three fairly arrays. VASP.4.4.2 (and newer versions) have a switch to run a more memory conserving symmetrization. This can be selected by specifying ISYM=2. Results might however differ somewhat from ISYM=1 (usually only 1/100th of an meV). Also avoid writing or reading the CHGCAR file (LCHARG=.FALSE.).

A final hint is in place. At some key places the code writes out the required memory per core. Please search the lines

total amount of memory used by VASP MPI-rank0

and inspect how much memory VASP uses per core.

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