VASP on the Clusters

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VASP and Slurm

In Slurm, there is big difference between --ntasks and --cpus-per-task which is explained in our Slurm Documentation (scroll down to Requesting Cores and Nodes)

For the purposes of VASP, --cpu-per-tasks should always equal NCORE (in your INCAR file). Then --ntasks should be equal to the total number of cores you want, divided by --cpu-per-tasks.

VASP has two parameters for controlling processor layouts, NCORE and NPAR, but you only need to set one of them. If you set NCORE, you don’t need to set NPAR. Instead VASP will automatically set NPAR to --ntasks. So the formula should be:

(--ntasks)*(--cpus-per-task) = total-number-of-cpus

and in your mpirun line, you should specify the total number of cpus:

mpirun -n total-number-of-cpus vasp_std 

You don’t need to specify —nodes unless you are trying to force the tasks on a certain number of nodes (which will likely increase you wait time with minimal speed up). But regardless, --nodes shouldn’t be part of the total number of cpu calculation.

Cores Layout Examples

If you want 40 cores (2 nodes and 20 cpus per node):

in your submission script:

#SBATCH --ntasks=2
#SBATCH --cpus-per-task=20

mpirun -n 40 vasp_std



You may however find that the wait time to get 20 cores on two nodes can be very long since cores request via --cpus-per-task can’t span multiple nodes. Instead you might want to try breaking it up into smaller chunks. Therefore, try:

in your submission script:

#SBATCH --ntasks=4
#SBATCH --cpus-per-task=10

mpirun -n 40 vasp_std



which would likely spread over 4 nodes using 10 cores each and spend less time in the queue.


On Omega, since cores are assigned as whole 8-core nodes, NCORE should always be equal to 8 and then you can just request —ntasks in multiples of 8.

in your submission script:

#SBATCH --ntasks=16 # some multiple of 8

mpirun -n $SLURM_NTASKS vasp_std



Additional Reading:

Here is some documentation on how to optimally configure NCORE and NPAR: