The ORCA quantum chemistry program package

TLDR

ORCA is a comprehensive general‑purpose quantum‑chemistry package used across chemistry, materials science, and biochemistry, with a specialty in transition‑metal chemistry and spectroscopy and a focus on real‑life applications to systems of a few hundred atoms. This contribution describes the ORCA program package. ORCA implements a broad range of electronic‑structure methods—including DFT, wave‑function correlation, semi‑empirical, and force‑field approaches—alongside solvation, embedding, QM/MM, and unique local‑correlation techniques based on pair natural orbitals. ORCA is widely adopted, with over 22,000 registered users in academia and industry.

Abstract

In this contribution to the special software-centered issue, the ORCA program package is described. We start with a short historical perspective of how the project began and go on to discuss its current feature set. ORCA has grown into a rather comprehensive general-purpose package for theoretical research in all areas of chemistry and many neighboring disciplines such as materials sciences and biochemistry. ORCA features density functional theory, a range of wavefunction based correlation methods, semi-empirical methods, and even force-field methods. A range of solvation and embedding models is featured as well as a complete intrinsic to ORCA quantum mechanics/molecular mechanics engine. A specialty of ORCA always has been a focus on transition metals and spectroscopy as well as a focus on applicability of the implemented methods to "real-life" chemical applications involving systems with a few hundred atoms. In addition to being efficient, user friendly, and, to the largest extent possible, platform independent, ORCA features a number of methods that are either unique to ORCA or have been first implemented in the course of the ORCA development. Next to a range of spectroscopic and magnetic properties, the linear- or low-order single- and multi-reference local correlation methods based on pair natural orbitals (domain based local pair natural orbital methods) should be mentioned here. Consequently, ORCA is a widely used program in various areas of chemistry and spectroscopy with a current user base of over 22 000 registered users in academic research and in industry.