diff --git a/source/lib/include/ewald.h b/source/lib/include/ewald.h
index bce151365a..654a6983e7 100644
--- a/source/lib/include/ewald.h
+++ b/source/lib/include/ewald.h
@@ -2,7 +2,12 @@
 
 #include<algorithm>
 #include<cassert>
+#if defined(_OPENMP)
 #include<omp.h>
+#else
+int omp_get_num_threads() { return 1; }
+int omp_get_thread_num() { return 0; }
+#endif
 
 #include "utilities.h"
 #include "region.h"