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SPARK/VisualSPARK allows users to construct models of complex building energy systems by connecting equation modules from a library. The program then uses robust methods to solve the resulting set of differential and algebraic equations at a user-specified time step. Current research includes work on a SPARK-to-EnergyPlus Link so that SPARK models of HVAC components, systems and controls can be run within EnergyPlus. The program is free of charge; register here to download.