ORCA

ORCA is a general-purpose quantum chemistry program package that features virtually all modern electronic structure methods. It is available on Aoraki as a module.

To make it available in your shell, load it with:

module load orca/6.1.0

This will add the ORCA executables to your path and set up the required environment variables.

Interactive session

An interactive session is useful for quick tests and debugging. The following command requests 4 CPUs, 16 GB of memory, and 1 hour of wall time on the aoraki partition:

srun --partition=aoraki --cpus-per-task=4 --mem=16G --time=01:00:00 --pty bash

Once connected to the compute node:

module load orca/6.1.0
orca water.inp > water.out

ORCA will automatically use the number of CPUs allocated to your job.

SLURM batch job

For longer calculations, use a SLURM batch job. Create an ORCA input file (e.g., water.inp) with your desired calculation settings.

Example ORCA input file:

! B3LYP def2-TZVP OPT FREQ
%pal nprocs 4 end
%maxcore 4000

* xyz 0 1
O  0.000000  0.000000  0.000000
H  0.757000  0.587000  0.000000
H -0.757000  0.587000  0.000000
*

Make sure the nprocs value matches the –cpus-per-task in your SLURM script, and set %maxcore appropriately for your memory allocation.

To submit a batch job, create a SLURM script (e.g., orca_job.slurm) with the following content:

#!/bin/bash
#SBATCH --job-name=orca_water
#SBATCH --partition=aoraki
#SBATCH --cpus-per-task=4
#SBATCH --mem=16G
#SBATCH --time=02:00:00
#SBATCH --output=%x-%j.out
#SBATCH --error=%x-%j.err

set -euo pipefail
module purge
module load orca/6.1.0


# Run ORCA
orca water.inp > water.out

# Copy important files back if needed
if [ -f "water.gbw" ]; then
    cp water.gbw "$SLURM_SUBMIT_DIR/"
fi

Submit the job with:

sbatch orca_job.slurm

Check the job status with:

squeue -u $USER

Monitor the calculation progress by checking the output file:

tail -f water.out

Best practices

Match resources: Set nprocs in your .inp file to match –cpus-per-task in your SLURM script.
Memory settings: Set %maxcore to about 80% of your total memory divided by the number of cores (in MB).
Scratch space: ORCA can generate large temporary files. The script above sets up a scratch directory.
File management: Copy important output files (.gbw, .xyz, .hess) back to your working directory.
Parallel efficiency: ORCA scales well up to 8-16 cores for most calculations.

Common ORCA keywords

DFT methods: B3LYP, PBE0, M06-2X, wB97X-D3
Basis sets: def2-SVP, def2-TZVP, def2-QZVP, cc-pVDZ, cc-pVTZ
Job types: OPT (optimization), FREQ (frequencies), SP (single point)
Advanced: CCSD(T), CASSCF, NEVPT2

Example input files

Single point energy calculation:

! B3LYP def2-TZVP
%pal nprocs 4 end

* xyz 0 1
C  0.000000  0.000000  0.000000
H  1.070000  0.000000  0.000000
H -0.356667  1.008971  0.000000
H -0.356667 -0.504485  0.874151
H -0.356667 -0.504485 -0.874151
*

Geometry optimization with frequencies:

! B3LYP def2-TZVP OPT FREQ
%pal nprocs 4 end
%maxcore 4000

* xyz 0 1
C  0.000000  0.000000  0.000000
O  1.200000  0.000000  0.000000
*