diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3528e4cc559f710c3469cdca5a2d1972f811a4b3..7362d9187874f090b1b22c27861c601fb6803ceb 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -70,7 +70,7 @@ if (OPENMP_FOUND)
     set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
     message(STATUS "OPEN MP ${OpenMP_CXX_SPEC_DATE}")
     if((OpenMP_CXX_SPEC_DATE GREATER_EQUAL "201511"))
-        set(CMAKE_CXX_FLAGS "-DOMP45")
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DOMP45")
         message(STATUS "Using OpenMP 4.5 directives")
     else()
         message(STATUS "Not using OpenMP 4.5 directives: ${OpenMP_C_VERSION_MAJOR} ${OpenMP_C_VERSION_MINOR}")