diff --git a/README.md b/README.md
index 2000a8a..285ff14 100644
--- a/README.md
+++ b/README.md
@@ -55,6 +55,7 @@ Thus, the Gauss's law must be solved in five-step interval for the discrete coor
 On the other hand, the relativistic formulae of velocity and momeutum
 /vec{v}= \vec{p}/(sqrt(m^2 +(px^2 +py^2 +pz^2)/c^2)) is valid
 in the nanoscale cases (Ref. 1 and Ref. 2).
+We insist that real mass of hydrogen, carbon and gold atoms, and electrons are used.
 Also, all the explicit simulation code must satisfy the Courant condition,
 that is, Dx(length)/Dt(time step) > c, the speed of light.
 Otherwise, a simulation is overflown quite shortly.