Dear professors,
Hello. I recently had to explain to my teacher the function of each part of the model I ran, but I couldn’t fully explain its function for the setting of background materials, just like the material model I set below. According to the requirement of ASPECT, my model sets two two-component fields. Therefore, the first parameter setting for each item is the background material, and the second and third items are the parameters of the compositional fields.
Best wishes,
Lu
Hi Lu,
Great question and thanks for posting it on the forum! Your description of how you need to define N + 1 material parameters when you do explicitly define N compositional fields is correct. ASPECT requires this background composition as a sort of fail-safe to ensure that the model has a material defined everywhere throughout the model domain. Imagine a case where the N user specified compositional fields only span a subset of the model domain, the background compositional field exists to populate the rest of the domain NOT covered by the N user defined compositions. If you were to define your N compositional fields so that they span the entire model domain, the background composition gets replaced by the user defined compositional fields, and has no impact on the model evolution.
I hope this clears this up!
Cheers,
Daniel
Thank you very much for your reply! 
Now I can explain to my teacher very well.
