Add Fortran API

[awvwgk]

Closes #66

[Copilot]

Pull request overview

This pull request adds Fortran/C bindings to GauXC to enable integration with legacy Fortran codes. The implementation introduces a C API layer and Fortran interfaces that bind to the C functions using iso_c_binding.

Changes:

• Added C API with headers and implementation for core functionality (molecule, basisset, matrix, integrator, etc.)
• Added Fortran modules that interface with the C API using iso_c_binding
• Updated build system to support optional C and Fortran API compilation via CMake options
• Added test coverage for C API interoperability in existing test files

Reviewed changes

Copilot reviewed 66 out of 66 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
CMakeLists.txt	Added GAUXC_ENABLE_C and GAUXC_ENABLE_FORTRAN options with build logic
src/CMakeLists.txt	Conditional compilation of C/Fortran sources based on enabled features
include/gauxc/*.h	C API headers for all major components (molecule, basisset, matrix, integrator, etc.)
src/c_*.cxx	C API implementation wrapping C++ classes
src/.f90, src/.F90	Fortran modules providing iso_c_binding interfaces to C API
include/gauxc/util/c_*.hpp	Helper utilities for C-C++ interop (pointer casting, conversions)
include/gauxc/gauxc_config.{h,f}.in	Configuration templates for C/Fortran feature detection
tests/moltypes_test.cxx	Added C API interoperability tests
src/external/c_hdf5_{read,write}.cxx	C API for HDF5 I/O operations
src/external/hdf5_{read,write}.f90	Fortran wrappers for HDF5 operations

Comments suppressed due to low confidence (1)

include/gauxc/gauxc_config.h.in:14

• The configuration macro GAUXC_HAS_C is defined in gauxc_config.f.in (line 1) but missing from gauxc_config.h.in. This creates an inconsistency where C/C++ code won't have access to the GAUXC_HAS_C macro to conditionally compile C API code, even though it's available in Fortran. Add #cmakedefine GAUXC_HAS_C to gauxc_config.h.in.

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[kjh-giesbertz]

Make sure that the c variant is called which transforms the fortran MPI communicator

Suggested change

	& result(rt) bind(c)
	& result(rt) bind(c, name="gauxc_runtime_environment_new_fortran")

[kjh-giesbertz]

Add some additional routines to querry info in basisset object. I found it useful to check during debugging.

Suggested change

	int32_t gauxc_basisset_nshells(GauXCStatus* status, const GauXCBasisSet basis) {
	status->code = 0;
	if (basis.ptr == nullptr) {
	status->code = 1;
	status->message = "No basis set associated to this object";
	return 0;
	}
	return detail::get_basisset_ptr(basis)->nshells();
	}
	int32_t gauxc_basisset_ngtos(GauXCStatus* status, const GauXCBasisSet basis) {
	status->code = 0;
	if (basis.ptr == nullptr) {
	status->code = 1;
	status->message = "No basis set associated to this object";
	return 0;
	}
	return detail::get_basisset_ptr(basis)->nbf();
	}

[kjh-giesbertz]

Interface the additional basis set routines also from fortran

Suggested change

	end interface
	integer(c_int32_t) function gauxc_basisset_nshells(status, basis) bind(c)
	import :: c_int32_t, gauxc_status_type, gauxc_basisset_type
	implicit none
	!> @param status Status of the operation
	type(gauxc_status_type), intent(out) :: status
	type(gauxc_basisset_type), value :: basis
	end function
	integer(c_int32_t) function gauxc_basisset_ngtos(status, basis) bind(c)
	import :: c_int32_t, gauxc_status_type, gauxc_basisset_type
	implicit none
	!> @param status Status of the operation
	type(gauxc_status_type), intent(out) :: status
	type(gauxc_basisset_type), value :: basis
	end function
	end interface

[kjh-giesbertz]

Add a variant which additionally transforms a fortran MPI communicator to the c-version

Suggested change

	GauXCRuntimeEnvironment gauxc_runtime_environment_new_fortran(
	GauXCStatus* status
	GAUXC_MPI_CODE(, MPI_Fint f_comm)
	) {
	MPI_Comm c_comm;
	c_comm = MPI_Comm_f2c(f_comm);
	return gauxc_runtime_environment_new(status GAUXC_MPI_CODE(, c_comm));
	}