The Xplor-NIH software package is a comprehensive tool for NMR biomolecular structure determination, built on the existing XPLOR program with additional NMR-specific extensions developed at the National Institutes of Health (NIH). The package includes a new C++ framework and interfaces with Python and TCL scripting languages, enabling rapid development of new tools and integration with external programs for structure analysis, manipulation, visualization, and spectral analysis. Key features of Xplor-NIH include: 1. **NMR-Specific Features**: Additional NMR-specific features not included in the original XPLOR, such as refinement against NMR observables, knowledge-based database potentials of mean force, and a reduced variable dynamics module. 2. **C++ Framework**: A new C++ framework for extending the functionality of Xplor-NIH, allowing for more flexible and efficient development of new features. 3. **Scripting Interfaces**: Interfaces with Python and TCL scripting languages, providing flexible alternatives to the legacy XPLOR scripting language and enabling integration with external tools. 4. **Potential Energy Terms**: A variety of potential energy terms for NMR experiments, including covalent geometry, non-bonded contacts, NMR observables, and knowledge-based potentials of mean force. 5. **Parameter and Topology Information**: Multiple parameter sets for proteins and nucleic acids, including those adapted for purely geometric refinements and full empirical energy functions. 6. **Regression Testing**: A suite of regression tests to ensure the proper functioning of the package, both in source and binary formats. 7. **Availability**: Source and binary versions of Xplor-NIH are available for noncommercial use, with commercial use arranged through Accelrys Corporation. The Xplor-NIH package aims to provide a robust and flexible tool for NMR biomolecular structure determination, leveraging modern programming languages and interfaces to enhance its capabilities and usability.The Xplor-NIH software package is a comprehensive tool for NMR biomolecular structure determination, built on the existing XPLOR program with additional NMR-specific extensions developed at the National Institutes of Health (NIH). The package includes a new C++ framework and interfaces with Python and TCL scripting languages, enabling rapid development of new tools and integration with external programs for structure analysis, manipulation, visualization, and spectral analysis. Key features of Xplor-NIH include: 1. **NMR-Specific Features**: Additional NMR-specific features not included in the original XPLOR, such as refinement against NMR observables, knowledge-based database potentials of mean force, and a reduced variable dynamics module. 2. **C++ Framework**: A new C++ framework for extending the functionality of Xplor-NIH, allowing for more flexible and efficient development of new features. 3. **Scripting Interfaces**: Interfaces with Python and TCL scripting languages, providing flexible alternatives to the legacy XPLOR scripting language and enabling integration with external tools. 4. **Potential Energy Terms**: A variety of potential energy terms for NMR experiments, including covalent geometry, non-bonded contacts, NMR observables, and knowledge-based potentials of mean force. 5. **Parameter and Topology Information**: Multiple parameter sets for proteins and nucleic acids, including those adapted for purely geometric refinements and full empirical energy functions. 6. **Regression Testing**: A suite of regression tests to ensure the proper functioning of the package, both in source and binary formats. 7. **Availability**: Source and binary versions of Xplor-NIH are available for noncommercial use, with commercial use arranged through Accelrys Corporation. The Xplor-NIH package aims to provide a robust and flexible tool for NMR biomolecular structure determination, leveraging modern programming languages and interfaces to enhance its capabilities and usability.