Atgeirr built and enhanced core simulation features across the OPM/opm-common and OPM/opm-simulators repositories, focusing on reliability, performance, and maintainability in C++ and Fortran. He refactored error handling by introducing standardized message formatting and improved diagnostics, which streamlined troubleshooting and reduced redundant warnings. In reservoir simulation modules, he optimized computational routines using OpenMP for parallel grid processing and introduced const-correctness to critical numerical methods, increasing safety and efficiency. Atgeirr also expanded documentation and user resources, and developed new simulation decks in OPM/opm-tests, broadening scenario coverage. His work demonstrated depth in scientific computing and petroleum engineering applications.