The ORCA quantum chemistry program package is a comprehensive tool for theoretical research in chemistry and related fields. Developed since the late 1990s, ORCA has evolved into a widely used software with over 22,000 registered users. It supports a range of methods including density functional theory (DFT), wavefunction-based correlation methods, semi-empirical methods, and even force-field methods. ORCA features a variety of solvation and embedding models, as well as a complete intrinsic quantum mechanics/molecular mechanics (QM/MM) engine. It is particularly focused on transition metals, spectroscopy, and real-life chemical applications involving systems with hundreds of atoms. ORCA is efficient, user-friendly, and platform-independent, with a number of unique methods. It includes linear- or low-order single- and multi-reference local correlation methods based on pair natural orbitals (domain based local pair natural orbital methods). ORCA is widely used in various areas of chemistry and spectroscopy. The program has a highly coherent code base that lends itself well to future development. ORCA has been developed with a focus on practical applications to real-life chemistry, biochemistry, and materials sciences. It has been used to solve concrete chemical problems where the questions to be addressed mostly originate from the experiment. ORCA features a range of spectroscopic and magnetic properties, and has been used to calculate electronic structure and optical spectral analysis of the MnO₄²⁻ anion. ORCA has been used to develop new integral libraries, such as SHARK, which significantly improves the performance of integral generation and digestion. ORCA has also been used to develop new methods, such as the DLPNO-CCSD(T) method, which is a linear scaling correlation method that has been widely used in computational chemistry. ORCA has also been used to develop new methods for excited states, such as the DLPNO-STEOM-CCSD method. ORCA has also been used to develop new methods for force fields, such as its own force field implementation. ORCA has also been used to develop new methods for geometry optimization, transition states, and minimum energy crossing points. ORCA has been used to develop new methods for the calculation of intermolecular interactions, such as the LED scheme. ORCA has been used to develop new methods for the calculation of dispersion energy, such as the HF-LD method. ORCA has been used to develop new methods for the calculation of multi-level schemes, such as the multi-level scheme. ORCA has been used to develop new methods for the integration of QM/MM approaches. ORCA has been used to develop new methods for the calculation of semi-empirical methods and force fields. ORCA has been used to develop new methods for the calculation of geometry optimization, transition states, and minimum energy crossing points. ORCA has been used to develop new methods for the calculation of intermolecular interactions, such as the LED scheme. ORThe ORCA quantum chemistry program package is a comprehensive tool for theoretical research in chemistry and related fields. Developed since the late 1990s, ORCA has evolved into a widely used software with over 22,000 registered users. It supports a range of methods including density functional theory (DFT), wavefunction-based correlation methods, semi-empirical methods, and even force-field methods. ORCA features a variety of solvation and embedding models, as well as a complete intrinsic quantum mechanics/molecular mechanics (QM/MM) engine. It is particularly focused on transition metals, spectroscopy, and real-life chemical applications involving systems with hundreds of atoms. ORCA is efficient, user-friendly, and platform-independent, with a number of unique methods. It includes linear- or low-order single- and multi-reference local correlation methods based on pair natural orbitals (domain based local pair natural orbital methods). ORCA is widely used in various areas of chemistry and spectroscopy. The program has a highly coherent code base that lends itself well to future development. ORCA has been developed with a focus on practical applications to real-life chemistry, biochemistry, and materials sciences. It has been used to solve concrete chemical problems where the questions to be addressed mostly originate from the experiment. ORCA features a range of spectroscopic and magnetic properties, and has been used to calculate electronic structure and optical spectral analysis of the MnO₄²⁻ anion. ORCA has been used to develop new integral libraries, such as SHARK, which significantly improves the performance of integral generation and digestion. ORCA has also been used to develop new methods, such as the DLPNO-CCSD(T) method, which is a linear scaling correlation method that has been widely used in computational chemistry. ORCA has also been used to develop new methods for excited states, such as the DLPNO-STEOM-CCSD method. ORCA has also been used to develop new methods for force fields, such as its own force field implementation. ORCA has also been used to develop new methods for geometry optimization, transition states, and minimum energy crossing points. ORCA has been used to develop new methods for the calculation of intermolecular interactions, such as the LED scheme. ORCA has been used to develop new methods for the calculation of dispersion energy, such as the HF-LD method. ORCA has been used to develop new methods for the calculation of multi-level schemes, such as the multi-level scheme. ORCA has been used to develop new methods for the integration of QM/MM approaches. ORCA has been used to develop new methods for the calculation of semi-empirical methods and force fields. ORCA has been used to develop new methods for the calculation of geometry optimization, transition states, and minimum energy crossing points. ORCA has been used to develop new methods for the calculation of intermolecular interactions, such as the LED scheme. OR