ORCA Quantum Chemistry: Install and Run Your First Calculation

Who this is for: a chemistry or materials researcher who has been told to “run it in ORCA” and needs to install the program and complete a first calculation. ORCA is a quantum chemistry program package developed at the Max-Planck-Institut fuer Kohlenforschung; its manual (checked 2026-11-12, version 6.1.1) describes it as “an ab initio, DFT and semiempirical SCF-MO package”. This guide follows the manual’s own quick-start path and adds a troubleshooting table drawn from the manual’s troubleshooting chapter.

What ORCA is used for in research

ORCA computes the electronic structure of molecules. FACCTs, the company that distributes commercial licences, describes it as a quantum chemistry package “primarily developed by the group of Prof. Frank Neese”, spanning semi-empirical methods, density functional theory (DFT), and correlated single- and multi-reference wavefunction methods. The manual’s chapters cover geometry optimisation, transition-state searches, vibrational frequencies and thermochemistry, excited states and TD-DFT, spectroscopy, NBO analysis, solvation, MP2, coupled cluster and CASSCF.

It is a command-line program. It reads a plain-text input file and writes its results to a text output file, so it suits scripted, cluster-based work. ORCA is free for academic use; commercial licences are available through FACCTs (see the FACCTs ORCA page). Check the licence terms on the registration page for your own situation before installing.

Install or access ORCA

ORCA is available for Linux, macOS and Windows. The installers are downloaded from the ORCA forum, which requires registration. Use that official route rather than third-party mirrors.

Linux and macOS

1. Download the .run installer for your platform (the manual’s example name is orca_6_1_0_linux_x86-64_shared_openmpi416.run).
2. Make it executable: chmod a+x orca_6_1_0_linux_x86-64_shared_openmpi416.run
3. Run it: ./orca_6_1_0_linux_x86-64_shared_openmpi416.run
4. The installer installs to a user directory and adds it to your PATH. Open a new terminal for the change to take effect.

Options go after a double dash: -i sets the install path interactively, -p <path> sets it directly, and -x only extracts the files. Example from the manual: ./orca_6_1_0_linux_x86-64_shared_openmpi416.run -- -p /my/home/orca/dir.

Windows

1. Unzip Orca6.1.0.Win64.zip and double-click Orca6.1.0.Win64.exe.
2. A typical install copies only the serial modules; choose Custom or Full to also install the parallel ones. The default directory is C:Orca_6.1.0.
3. The installer sets the environment variables PATH, ORCADIR and XTBEXE. They affect only command windows opened afterwards.
4. The AUTOCI modules are separate zip files (serial, and an msmpi10 parallel build) that you extract into the install directory.

On Linux and Windows, ORCA 6 bundles xtb 7.6.1.

Step-by-step: your first calculation

The manual’s first example is a Hartree-Fock calculation on molecular hydrogen. The input is free-format ASCII, normally saved with the .inp extension.

1. Create orca.inp containing:

   ! HF DEF2-SVP
   *XYZ 0 1
   H   0  0  0
   H   0.74  0  0
   *

2. Read the input: the line starting with ! is the “simple input line” holding the method (HF) and basis set (def2-SVP). The block opened by *XYZ gives the charge (0) and the multiplicity (1, where multiplicity M = 2S+1), then one atom per line with Cartesian coordinates in Angstrom. A closing * ends the block.
3. Run it, redirecting output because ORCA writes to standard output and error. On Windows: orca orca.inp > orca.out. On Unix: orca orca.inp >& orca.out &
4. Follow progress on Unix with tail -f orca.out.

The manual’s next example, “Hello water”, uses the same method on water (hellowater.inp with an O atom at 0.0000 0.0000 0.0626 and H atoms at -0.7920 0.0000 -0.4973 and 0.7920 0.0000 -0.4973) and is run with orca hellowater.inp > hellowater.out.

Output and how to read it

• Did it finish? A normal run ends with the message ****ORCA TERMINATED NORMALLY**** at the end of the output file. If it is absent, read the last lines of the output to see why the run stopped.
• The energy. The final energy appears on the line FINAL SINGLE POINT ENERGY. For the manual’s water example (HF/def2-SVP) it is -75.959340303174 (Hartree is the program’s standard energy unit).
• Files left behind. orca.gbw (Geometry-Basis-Wavefunction) is a binary file needed to restart an SCF or to begin from existing orbitals. The orca*.tmp files are intermediates, deleted on normal exit; after a crash remove them yourself (rm orca*.tmp on Unix, del orca*.tmp on Windows).
• Do not overwrite the output file while a calculation is still running.

Troubleshooting

Symptom	What the manual says to check
Command not found, or the wrong program starts	Check PATH with which orca (Unix) or where orca (Windows). Another program called ORCA may interfere; call ORCA by its full path.
Error “ORCA has to be called with full pathname”	Parallel runs require ORCA to be called by its full pathname.
Parallel run misbehaves	Do not start the main executable under mpirun. The driver runs serially and calls the parallel modules itself.
No output appears	The output was not redirected to a file.
“UNRECOGNIZED OR DUPLICATED KEYWORD(S) IN SIMPLE INPUT LINE” after setting cores	The simple keywords are only PAL2 to PAL8, PAL16, PAL32 and PAL64; for example PAL12 is not accepted.

The manual’s troubleshooting chapter also has sections on SCF convergence, running out of memory, full disks, linear dependence and imaginary frequencies.

Graphical interfaces

ORCA has no native GUI. The manual lists third-party programs, none developed by the ORCA team: Avogadro 2 (build, create input, visualise), ChimeraX (visualise) and Chemcraft (build, create input, visualise; Windows and Linux).

Free alternatives

Program	Description from its own site	Licence
Psi4	Open-source quantum chemistry program with a C++ core and Python layer; optimised for multi-core DFT, MP2, SAPT and coupled-cluster calculations	LGPL3
PySCF	Python quantum chemistry package for molecules and periodic solids; modular and extensible	Apache 2.0

Pages checked 2026-11-12. For related tool guides on this site, see molecular dynamics simulation, computational materials science journals, HPC allocation options, the research software engineer role and the Software Citation FAIR working group; cite the ORCA version you used when you publish results.

FAQ

What is ORCA quantum chemistry software?

ORCA is an ab initio, DFT and semiempirical SCF-MO package for computing molecular electronic structure, developed primarily in the group of Prof. Frank Neese at the Max-Planck-Institut fuer Kohlenforschung and distributed with commercial licences through FACCTs (FACCTs page and ORCA 6.1.1 manual, checked 2026-11-12).

Is ORCA free?

FACCTs states that ORCA is free for academic use and that commercial licences are available through FACCTs.

Which operating systems does ORCA run on?

Linux, macOS and Windows, per the manual’s installation chapter.

Does ORCA have a graphical interface?

Not natively. Avogadro 2, ChimeraX and Chemcraft are third-party tools listed in the manual.

Sources

• ORCA 6.1.1 Manual, Max-Planck-Institut fuer Kohlenforschung: orca-manual.mpi-muelheim.mpg.de (installation, running, Hello water, GUI and troubleshooting pages), read 2026-11-12.
• FACCTs, ORCA: faccts.de/orca, read 2026-11-12.
• Psi4: psicode.org; PySCF: pyscf.org, read 2026-11-12.