diff --git a/include/constants.hpp b/include/constants.hpp
index 887e20b..87f1e95 100644
--- a/include/constants.hpp
+++ b/include/constants.hpp
@@ -12,4 +12,8 @@
 #ifndef __CONST__
 #define __CONST__
 
+/** @brief Boltzmann constant. unit \f$ \frac{J}{K} \f$.
+ * */
+#define k_B 1.380649e-23 
+
 #endif