--- a/configure.ac
+++ c/configure.ac
@@ -440,7 +440,7 @@ else
     FFLAGS=${I_BASE}" -Ofast "${I_ARCH}
     
   elif test x"${FC_ACT}" = x"gfortran"; then
-    FFLAGS=${G_BASE}" -O3 "${G_ARCH}
+    FFLAGS=${G_BASE}" -O3 -fopenmp "${G_ARCH}
     
   elif test x"${FC_ACT}" = x"mpifrtpx"; then
     FFLAGS=${F_BASE}" -Kfast,ocl,simd,swp "${F_ARCH}
@@ -465,7 +465,7 @@ else
     FCFLAGS=${I_BASE}" -Ofast "${I_ARCH}
     
   elif test x"${FC_ACT}" = x"gfortran"; then
-    FCFLAGS=${G_BASE}" -O3 "${G_ARCH}
+    FCFLAGS=${G_BASE}" -O3 -fopenmp "${G_ARCH}
     
   elif test x"${FC_ACT}" = x"mpifrtpx"; then
     FCFLAGS=${F_BASE}" -Kfast,ocl,simd,swp "${F_ARCH}
@@ -486,7 +486,7 @@ if   test x"${FC_ACT}" = x"ifort"; then
   FFLAGS0=${I_BASE}" -O0 "
   
 elif test x"${FC_ACT}" = x"gfortran"; then
-  FFLAGS0=${G_BASE}" -O0"
+  FFLAGS0=${G_BASE}" -O0 -fopenmp "
   
 elif test x"${FC_ACT}" = x"mpifrtpx"; then
   FFLAGS0="-Kopenmp,openmp_ordered_reduction -Cpp"