# ORCA¶

ORCA is a computational chemistry code that can do SCF, (TD)DFT, semi-empirical potentials, MP2, CASSCF, Coupled Cluster calculations, and more.

It is closed source, but free for academic users. Register on the forum to receive a download link for the binaries, as well as access ot the latest manual.

Many input examples are available at the ORCA Input Library.

The ORCA ASE-interface is very simple. Two keywords are defined:

orcasimpleinput: str
What you'd put after the "!" in an orca input file.

orcablock: str
What you'd put in the "% ... end"-blocks.


The ASE-calculator also works with the EIQMMM-calculator for QM/MM simulations (see qmmm for more info).

## Setup and usage¶

The default command that ASE will use to start ORCA is orca PREFIX.inp > PREFIX.out.

You can change this command by setting the environment variable \$ASE_ORCA_COMMAND. (For example, add a line to your .bashrc with export ASE_ORCA_COMMAND="my new command"). This can be useful since the parallel MPI version of orca can require the full path to the executable to be specified.

Orca wants to decide which sub-processes to parallelize via MPI itself, so you’d almost always want a string in your orcablocks specifying the number of cores for the simulation, e.g.:

from ase.calculators.orca import ORCA

calc = ORCA(label='orcacalc',
orcasimpleinput='B3LYP def2-TZVP'
orcablocks='%pal nprocs 16 end'


for a B3LYP/def2-TZVP calculation on 16 cores.