Thank you for the suggestions B.R.

First of all, the calculations are fairly complex. The only way that I
can describe them generally is to say that they have the form of a
matrix equation and the solution is the adjustments on the two initial
values. These values are added to the initial values (input) to yield
the final values (output).

I am having trouble adapting your first suggestion to my calculations,
but I will keep trying.

Solver failed to converge using any of the values that I tried. I
understand that the initial estimates can have an impact on
convergence, but I am simply trying to replicate a calculation that has
been demonstrated in literature repeatedly. Therefore, I have reason
to believe the initial values that I have chosen are adequate.

Do you have any idea what the difference is between the macro and
excel's iteration?

thanks for your help