The ORCA quantum chemistry program package is a comprehensive and versatile tool for theoretical research in chemistry and related fields. Developed by Frank Neese, Frank Wennmohs, Ute Becker, and Christoph Ripinger, ORCA has grown from a semi-empirical package for biochemical problems to a widely used general-purpose program with over 22,000 registered users. Key features include density functional theory (DFT), wavefunction-based correlation methods, semi-empirical methods, and force-field calculations. ORCA emphasizes practical applications, particularly in transition metal chemistry, spectroscopy, and magnetism. It supports a wide range of basis sets, integral approximations, and molecular dynamics simulations. Notable contributions include the development of linear scaling local correlation methods based on pair natural orbitals (PNOs) and the implementation of the domain-based local pair natural orbital (DLPNO) approach for coupled cluster theory. ORCA also offers robust geometry optimization, transition state search, and solvation models, making it a powerful tool for both academic and industrial research.The ORCA quantum chemistry program package is a comprehensive and versatile tool for theoretical research in chemistry and related fields. Developed by Frank Neese, Frank Wennmohs, Ute Becker, and Christoph Ripinger, ORCA has grown from a semi-empirical package for biochemical problems to a widely used general-purpose program with over 22,000 registered users. Key features include density functional theory (DFT), wavefunction-based correlation methods, semi-empirical methods, and force-field calculations. ORCA emphasizes practical applications, particularly in transition metal chemistry, spectroscopy, and magnetism. It supports a wide range of basis sets, integral approximations, and molecular dynamics simulations. Notable contributions include the development of linear scaling local correlation methods based on pair natural orbitals (PNOs) and the implementation of the domain-based local pair natural orbital (DLPNO) approach for coupled cluster theory. ORCA also offers robust geometry optimization, transition state search, and solvation models, making it a powerful tool for both academic and industrial research.