I’ve completely reinstalled my OS and am now encountering issues that I’ve heard other talk about, I’ve followed the instructions on the website word by word, except that I forked Michael’s repo and changed to kwant version 1.0.3. This has worked before for me. This is the error message: brew install kwant ==> Installing kwant from basnijholt/homebrew-kwant ==> Using Homebrew-provided fortran compiler. This may be changed by setting the FC environment variable. ==> Building with an alternative Fortran compiler This is unsupported. Warning: No Fortran optimization information was provided. You may want to consider setting FCFLAGS and FFLAGS or pass the `--default-fortran-flags` option to `brew install` if your compiler is compatible with GCC. If you like the default optimization level of your compiler, ignore this warning. ==> Downloading pypi.python.org/packages/source/k/kwant/kwant-1.0.3.tar.gz Already downloaded: /Library/Caches/Homebrew/kwant-1.0.3.tar.gz ==> Patching patching file build.conf ==> python setup.py install --prefix=/usr/local/Cellar/kwant/1.0.3 Build configuration was: User-configured LAPACK and BLAS User-configured MUMPS ********************************************************************** error: command 'clang' failed with exit status 1 READ THIS: https://git.io/brew-troubleshooting If reporting this issue please do so at (not Homebrew/homebrew): https://github.com/basnijholt/homebrew-kwant/issues Best Bas