>    (define (eps-func p)
>      (let* ((lattice->cartesian p)
>        ...
>
> You probably want to change this to
>
> (define (eps-func p)
>   (let* ((p (lattice->cartesian p))
>     ...
>

This is really make sense to me! Now p is the cartesian coordinate, and
then we can make values for x, y, and z. And it is also reasonable for the
LET statement.

I will try it and see the results, but it takes a long time to run the
program.

Thank you very much, Campbell.

Regards,

Simon