+#define ONE_THIRD (1.0/3.0)
+
+/*
+ * default values
+ *
+ * - length unit: 1 A (1 A = 1e-10 m)
+ * - time unit: 1 fs (1 fs = 1e-15 s)
+ * - mass unit: 1 amu (1 amu = 1.6605388628e-27 kg )
+ *
+ * fyi: in the following 1 N = (amu*A)/(fs*fs)
+ *
+ */
+
+#define METER 1e10 /* A */
+#define SECOND 1e15 /* fs */
+#define AMU 1.6605388628e-27 /* kg */
+#define KILOGRAM (1.0/AMU) /* amu */
+#define NEWTON (METER*KILOGRAM/(SECOND*SECOND)) /* A amu / fs^2 */
+#define PASCAL (NEWTON/(METER*METER)) /* N / A^2 */
+#define ATM (1.0133e5*PASCAL) /* N / A^2 */