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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:

Terminal

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:

Terminal

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

Once connected to the compute node:

Terminal

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:

```text

! 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:

Terminal

#!/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:

Terminal

sbatch orca_job.slurm

Check the job status with:

Terminal

squeue -u $USER

Monitor the calculation progress by checking the output file:

Terminal

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:

```text
! 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:

```text

! 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
*
```

See also