Short summary:
This repository contains two Fortran implementations of the Appendix B example used for HeH⁺ SCF.

• AppendixBcode.f90 original, Fortran-77 style reference source.
• AppendixBcode_revised.f90 Fortran-90 refactor by the author that preserves original variables and algorithms, rewrites control flow (removes GOTOs; uses DO/IF), and performs diagonalization using LAPACK.

The file AppendixBcode_revised.f90 includes the following major modifications relative to AppendixBcode.f90:

1. Removed jumps and rewrote control flow
   All unstructured jumps (e.g., GOTO) were removed and rewritten using structured DO and IF blocks.

2. Retained original variables and integral calculations
   The original variable definitions and electronic integral evaluation procedures were preserved to allow direct comparison with the reference code.

3. LAPACK-based diagonalization
   Matrix diagonalization is performed using LAPACK routines (e.g., DSYEV).
   Some variables are redundant and have intentionally not been removed to facilitate validation against the original implementation.

• Fortran compiler supporting Fortran-90
• LAPACK library (and BLAS, depending on system configuration)

gfortran -o HeH AppendixBcode_revised.f90 -llapack