Hi all, I have attached a notebook which I am doing a calculation in. For some reason, Mathematica is acting inconsistently. When I run the calculation for the first time, it usually doesn't work. By that I mean I get the output that you can see in the notebook attached. You can that the problem is coming from the subscript 3 in the term H3. I can't understand why it doesn't just read the value of H3 I input early on!? I could just ditch the subscript but I really don't want to because I want to eventually change the subscript 3 for a general integer i and rerun the program for many different i values. I should also add that sometimes the program runs with no problems despite having changed nothing. Once it runs once OK, then everytime thereafter is OK. Please can someone explain what on Earth is going on!