work around race condition and minimizing download attemps for transparently...

      math(EXPR plusone "${blank}+1")

      string(SUBSTRING ${line} 0 ${blank} pot)

      string(SUBSTRING ${line} ${plusone} -1 sum)

      if(EXISTS ${LAMMPS_POTENTIALS_DIR}/${pot})

        file(MD5 "${LAMMPS_POTENTIALS_DIR}/${pot}" oldsum)

      endif()

      if(NOT sum STREQUAL oldsum)

        message(STATUS "Checking external potential ${pot} from ${LAMMPS_POTENTIALS_URL}")

      file(DOWNLOAD "${LAMMPS_POTENTIALS_URL}/${pot}.${sum}" "${LAMMPS_POTENTIALS_DIR}/${pot}"

        file(DOWNLOAD "${LAMMPS_POTENTIALS_URL}/${pot}.${sum}" "${CMAKE_BINARY_DIR}/${pot}"

          EXPECTED_HASH MD5=${sum} SHOW_PROGRESS)

        file(COPY "${CMAKE_BINARY_DIR}/${pot}" DESTINATION ${LAMMPS_POTENTIALS_DIR})

      endif()

    endforeach()

  endif()

endfunction(FetchPotentials)