The backend of CalcUS is written in the programming language Python, which is widely used in science. As such, many Python packages can be utilized directly to provide useful features. The general software infrastructure is presented in Scheme 1. Every software component and module used are open-source and available freely under various permissive licenses. The program is centered around a Django web application, which powers the interface. The web server handles the requests and manages the data stored in a PostgreSQL database. Tasks which have long runtimes, such as local calculations, are sent to a queue and processed by the task manager. Remote connections to computation clusters are handled by a separate daemon, which controls file upload, task monitoring and result analysis.