Geant4 does not find Expat library

Solution 1:

From the Geant4 documentation:

GEANT4_USE_SYSTEM_EXPAT (DEFAULT : ON)

If set to ON, build Geant4 with an external install of Expat. In this case, the Expat headers and library will be searched for under the standard system paths. If these are not sufficient to locate the required Expat installation, see the Advanced EXPAT_INCLUDE_DIR and EXPAT_LIBRARY options.

Whilst Expat is installed on the vast majority of systems, it may be missing in certain instances. In these cases, simply switch this option to OFF and Geant4 will build and use its internal version of Expat.

REQUIRES : Expat library and headers.