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# 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.