diff --git a/src/MOFFF/improper_inversion_harmonic.cpp b/src/MOFFF/improper_inversion_harmonic.cpp
index c0de968626..681811594b 100644
--- a/src/MOFFF/improper_inversion_harmonic.cpp
+++ b/src/MOFFF/improper_inversion_harmonic.cpp
@@ -332,3 +332,15 @@ void ImproperInversionHarmonic::write_data(FILE *fp)
   for (int i = 1; i <= atom->nimpropertypes; i++)
     fprintf(fp,"%d %g %g\n",i,kw[i],w0[i]/MY_PI*180.0);
 }
+
+/* ----------------------------------------------------------------------
+   return ptr to internal members upon request
+------------------------------------------------------------------------ */
+
+void *ImproperInversionHarmonic::extract(const char *str, int &dim)
+{
+  dim = 1;
+  if (strcmp(str, "kw") == 0) return (void *) kw;
+  if (strcmp(str, "w0") == 0) return (void *) w0;
+  return nullptr;
+}