As you may have noticed, the CHARMM documentation is not very complete or up to date.
Here's what I've been doing successfully for years, and I just tested this on OS X Catalina:
- obtain gcc from Xcode; verify via 'which gcc'
- obtain and install gfortran (only) for your OS X version from hpc.sourceforge.net ; verify via 'which gfortran'
- obtain and install CMake for OS X from cmake.org
- on the 'configure' command line, include '--cmake /Applications/CMake.app/Contents/bin/cmake'
I usually do cmake builds in an empty parallel directory on OS X, so I invoke configure via e.g. ../c45b2/configure --cmake /Applications/CMake.app/Contents/bin/cmake
and then follow with 'make -j8'
Before configure, I used the first two steps above with the install.com build procedure, which usually worked.Update for Big Sur
For OS X 11 (Big Sur) I had some problems initially, but after some updates to Xcode and the GCC suite at hpc.sourceforge.net, this minimalist approach is working again. The one change I had to make was to install the full GCC suite, instead of just gfortran, and use those C and C++ compilers (gcc, g++) for the CHARMM build. Xcode is only needed for the final link step, and these GCC compilers do that automatically.