A handful of forcefields ready-formatted for import into Aten are provided with the code and are listed here. It should be a relatively straightforward process to convert others, unless the functional forms used are not yet implemented (but sure, if you ask then I will add them). If you export an expression from Aten, please check the parameters in the file are what you actually want. Aten is designed to ease the pain of setting up a simulation in this manner, but is not intended as a black box.