From my experience with the ODE solvers, I don’t see any reason that you could not do what you want. However you don’t need the ‘@’ sign if you’re calling an external function and are not passing the function as an argument to another function. Just call it as you would any other function (for instance sin(y(1))):
dy(5) = a*y(4) + b*y(3)*y(4) + heatofVap(y(1))*y(3);
You’ve likely provided as good an example of using it as I could come up with!
EDIT — It is always good practise to vectorise your code, to be certain you are doing element-wise operations rather than matrix operations (unless you specifically intend to do matrix operations). I would change the ‘dy(5)’ and other assignments to use element-wise operations:
dy(5) = a.*y(4) + b.*y(3).*y(4) + heatofVap(y(1)).*y(3);
