The Research Support Facility at the National Renewable Energy Laboratory is a 220,000-ft2 office building designed to serve 822 occupants, to use 35.1 kBtu/(ft2·yr), to use half the energy of an equivalent minimally code-compliant building, and eventually to produce as much renewable energy annually as it consumes. These goals and their substantiation through simulation were explicitly included in the fixed price design-build contract. The energy model had to be repeatedly updated to match design documents and the final building, as it was built, to the greatest degree practical. Computer modeling played a key role in diagnosing the energy impacts of program and decisions and in verifying that the contractual energy goals would be met within the specified budget. The primary tool used was a whole-building energy simulation program. Other simulation tools were used to provide more detail or to complement the primary tool as required by the delivery schedule, including tools to calculate thermal bridging, daylighting, natural ventilation, data center energy consumption, transpired solar collectors, thermal storage in the crawlspace, and electricity generation by photovoltaic panels. Results were either fed back into the main whole-building energy simulation tool or used to post-process model output to provide the most accurate annual simulations possible. This paper details the models used in the design process and how they informed important program and design decisions from design to completion.