There are a number of reasons for the crash. When initialization occurs, variables are evaluated, so this is where problems with UDFs can sometimes make an early appearance. The possibilities are: 1. The routine was interpreted, and the interpretation cannot be processed by Fluent because the UDF is too complex. If this is the case, compiling the routine should eliminate the problem. 2. An inappropriate UDF was used to define the property. Some properties, like Specific Heat, have their own unique DEFINE routine. If Specific heat is defined with DEFINE_PROPERTY rather than DEFINE_SPECIFIC_HEAT, Fluent will crash.