Do we need the code duplication of blas.f90 and blas64.f90 here? It would be preferable if we can just have a single file rather than two to avoid divergence.

It is not necessary. Let me remove blas64.f90 and work with macros

You could define a kind parameter ik and use it instead of i8

For the macro I would go with something along the lines of

#ifndef IK
#define IK i4
#endif

module mchrg_blas
  use mctc_env, only : sp, dp, ik => IK
  implicit none

  ! ...

end module mchrg_blas

Let me know if this works for you. If this is ok with you, I would also prepare PR's for dftd4, s-dftd3, and tblite with the same structure to enable ILP64

Actually, I was thinking the macros are not needed. If someone (like me) wants to use the ILP64 interface, then one can handle that with the -Dilp64=true and giving the right fortran flags, i.e. -Dfortran_flags="-fdefault-integer-8.

What's your preference @awvwgk ? I can close this PR and open a new one with a clean commit history

Indeed, promoting default integers to 8 bytes (-i8 for Intel and -fdefault-integer-8 for GCC) and using the custom library option should make this possible without changes in the build files already.

However, I'm still open to officially support ilp64 with the tblite stack as well.

If the C compiler is needed for MKL detection, we could add it optionally in Findcustom-blas.cmake, if the correct BLA_VENDOR is set.

The following works for either LP64 or ILP64 if cmake >= 3.22 and $MKLROOT is set. For older versions of cmake the user should provide the correct BLA_VENDOR

diff --git a/config/cmake/Findcustom-blas.cmake b/config/cmake/Findcustom-blas.cmake
index 3dfe5c1..7f71e62 100644
--- a/config/cmake/Findcustom-blas.cmake
+++ b/config/cmake/Findcustom-blas.cmake
@@ -13,6 +13,14 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+if ((BLA_VENDOR MATCHES ^Intel) OR (DEFINED ENV{MKLROOT}))
+  enable_language("C")
+endif()
+
+if(WITH_ILP64)
+  set(BLA_SIZEOF_INTEGER 8)
+endif()
+
 if(NOT BLAS_FOUND)
   find_package("BLAS")
   if(NOT TARGET "BLAS::BLAS")
diff --git a/config/cmake/Findcustom-lapack.cmake b/config/cmake/Findcustom-lapack.cmake
index c78569d..8cd9a6a 100644
--- a/config/cmake/Findcustom-lapack.cmake
+++ b/config/cmake/Findcustom-lapack.cmake
@@ -13,6 +13,14 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+if ((BLA_VENDOR MATCHES ^Intel) OR (DEFINED ENV{MKLROOT}))
+  enable_language("C")
+endif()
+
+if(WITH_ILP64)
+  set(BLA_SIZEOF_INTEGER 8)
+endif()
+
 if(NOT LAPACK_FOUND)
   find_package("LAPACK")

This seems like a good solution to me.