ORCA is a scientific software that provides cutting-edge methods in the fields of density functional theory and correlated wave-function based methods.

Basic Usage

You can do a simple run with ORCA with the following script.

#SBATCH --time=00:15:00
#SBATCH --nodes=1
#SBATCH --ntasks-per-node=4
#SBATCH --mem=2G
#SBATCH --output=orca_example.out

module load orca/4.2.1-openmpi

rm -f water*

cat > water.inp << EOF
* xyz 0 1
O   0.0000   0.0000   0.0626
H  -0.7920   0.0000  -0.4973
H   0.7920   0.0000  -0.4973

# Parallel runs need the full path to orca executable
# Do not use srun as orca will call mpi independently: https://www.orcasoftware.de/tutorials_orca/first_steps/trouble_install.html#using-openmpi
$(command -v orca) water.inp

This script performs a parallel run of ORCA to simulate the behavior of water molecule. The input file for this simulation is called water.inp, which is written to by the cat command.

To run this script, download it and submit into the queue using sbatch:

$ wget https://raw.githubusercontent.com/AaltoSciComp/scicomp-docs/master/triton/examples/orca/orca_example.sh
$ sbatch orca_example.sh

How to launch ORCA when using MPI parallelism

When doing parallel runs you should always launch ORCA with

$(command -v orca) input_file.inp

in your Slurm scripts. This is because ORCA will need the executable to be launched with the full path of the executable and it will launch the MPI tasks independently. For more information, see this documentation page.

Setting the number of MPI tasks

The example given above asked for 4 MPI tasks by setting #SBATCH --ntasks-per-node=4 in the Slurm batch script and then told ORCA to use 4 tasks by setting !PAL4 in the input file.

When asking for more than 8 tasks you need use %PAL NPROCS 16 END to set the number of tasks in ORCA input (here, the line would specify 16 tasks).

For more information please refer to ORCA’s documentation page on parallel calculations.