ORCA
User Guide

A practical English companion to the ORCA quantum chemistry package, written against the 6.0.0 manual — input recipes and notes on reading the output, for the calculations you actually run.

Version 6.0.0 · Supports 5.0.2 / 5.0.4 / 6.0.0 · Based on Manual 2024.07

About this guide

ORCA covers a wide span of quantum chemistry in one program — from single-point energies and geometry optimisations, through frequencies and thermochemistry (free energies), transition states and reaction paths (NEB · IRC), excited states and UV-Vis spectra (TD-DFT), on to high-level correlated and multireference methods (CASSCF, NEVPT2, DLPNO-CCSD(T)) and magnetic / spectroscopic properties such as NMR and EPR. It is especially strong in transition-metal chemistry and spectroscopy, and free for academic use. This guide distils the parts of the 1300-page manual you reach for most often, written for practical use.

The left sidebar jumps directly to any topic, and every page ends with previous / next links so the guide can also be read end-to-end. All input examples target ORCA 6.0.0; the vast majority of them work unchanged on the 5.0.x line, and any change that affects backward compatibility is called out explicitly.

Browse by topic

If you are just starting

01 · GETTING STARTED
Getting started
Check the installation, run your first job, and tour the output files.
02 · INPUT
Input file structure
Keyword line, %-blocks, and coordinate input on a single page.
03 · KEYWORDS
Keywords
A short dictionary of the most common method, basis, and convergence keywords.

The common calculations

04 · SINGLE POINT
Single point energy
Energy and wavefunction at a fixed geometry — the most basic job.
05 · OPTIMISATION
Geometry optimisation
Constraints, choice of coordinates, and 1D / 2D potential surface scans.
06 · FREQUENCIES
Frequencies · Thermochemistry
Analytical vs numerical frequencies, and Gibbs free energy from the output.
07 · TS · NEB
Transition states · NEB
NEB-TS, ScanTS, and other strategies for locating activation barriers.
08 · EXCITED STATES
Excited states · TD-DFT
UV-Vis absorption spectra and the basics of fluorescence calculations.
09 · SOLVATION
Solvation
Practical use of implicit-solvent models such as CPCM and SMD.

Choosing a method and running it well

10 · CC
DLPNO-CCSD(T)
The gold-standard correlation method, now affordable for large molecules.
11 · BASIS
Basis sets
When to reach for def2, cc-pVnZ, SARC, and friends.
12 · DFT
DFT functionals
Beyond B3LYP — r²SCAN, ωB97X-V and others.
13 · CASSCF
CASSCF · Multireference
Where single-reference breaks — bond dissociation, transition-metal magnetism.
14 · TIPS
Tips & troubleshooting
SCF convergence failure, out-of-memory errors, and other everyday traps.

On citing ORCA

When publishing work that used ORCA, please include the three general citations below. On top of these it is customary to also cite the original method papers for whichever technique you actually used (e.g. DLPNO-CCSD(T), NEB-TS, CASSCF, …).

  • Neese, F. WIREs Comput. Mol. Sci. 2012, 2, 73–78.
  • Neese, F. WIREs Comput. Mol. Sci. 2018, 8, e1327.
  • Neese, F.; Wennmohs, F.; Becker, U.; Riplinger, C. J. Chem. Phys. 2020, 152, 224108.
Reference material

Everything in this guide is anchored to orca_manual_6_0_0.pdf (2024-07-24 edition). For deeper options or unusual use cases please consult the manual directly. The official ORCA forum (orcaforum.kofo.mpg.de) is also very useful when searching for concrete real-world examples.