Many technological advances in modern-day life depend upon the development of new materials, or better control and understanding of existing materials. The chemical, mechanical and physical properties of materials depend on their constituent atoms and, in particular, their electrons. CASTEP is a state-of-the-art software package which uses quantum mechanics to predict the behaviour of those electrons and, hence, the material, and it is widely used by scientists in academia and industry. Many of these researchers are experimental scientists, rather than computational specialists, and the main aim of this proposal is to support them to use CASTEP more easily, efficiently and reliably, and to expand the user community by lowering the barrier of entry for new users. The work focuses on preparing CASTEP for the future, by improving its Usability, Sustainability, Efficiency and Reliability (USER) so any researcher can run it quickly, consistently and easily on any computer, from laptops to HPC facilities. The key challenges this proposal addresses are to: * enhance accessibility for non-specialist scientists * exploit future methods and technologies * take full advantage of available computing resources * further improve reliability, and be fully validated This far-reaching programme will improve the whole CASTEP user experience, including: re-imagining CASTEP's interface (focusing on scientific output, not algorithmic details) and creating comprehensive examples and tutorials; developing a deep API for embedding CASTEP in high-level workflows; automating CASTEP's parallel decomposition; and improving fault-tolerance. The work will be in collaboration with consortia (e.g. MCC, UKCP, CCP-NC, CCP9) and national experimental facilities (e.g. SuperSTEM), as well as industry partners (e.g. NVIDIA and BIOVIA). The ultimate, overarching goal is that CASTEP itself becomes 'invisible'; a hidden software infrastructure for providing quick, clear answers to research questions, whose correctness and successful operation may be taken for granted. The research described in this proposal will make significant impacts on many areas of academic and industrial research, particularly in materials for future technology. CASTEP is already used by well over 1000 academic groups and industrial research sites across the globe, including Johnson Matthey, Sony, Solvay, PG Corp, Pfizer, Astra Zeneca and Toyota, and supports research in a vast range of materials such as semiconductor nanostructures, ultra-high temperature ceramics, nanoscale devices, fluorophores, thermoelectrics, hybrid perovskites and solar cells, inorganic nanotubes and metal-air battery anodes. This work will promote CASTEP use across a diverse range of STEM disciplines, increase the effectiveness and impact of a wide variety of research initiatives, and enable researchers to directly address 5 of EPSRC's Grand Challenges in Physics, Engineering and Chemical Science.