I have a scalar UDF that I know to be deterministic, however SQL doesn't. Is there a way to declare it as deterministic so that I can then use it in a persisted computed column definition?

further clarification: The purpose of this exercise is that I need to harvest out specific values from an XML column on the row. I can't use the value method of the xml column in my computed column definition, but I can use it in a UDF. I know the xpath query in the value method will produce the same output give the same input so while I certainly understand that not all calls to value will be deterministic I want to assert that mine is.