diff --git a/src/PenningTrap.cpp b/src/PenningTrap.cpp
index 078f17d..2ebc667 100644
--- a/src/PenningTrap.cpp
+++ b/src/PenningTrap.cpp
@@ -105,7 +105,7 @@ arma::vec PenningTrap::total_force_particles(int i)
 
 arma::vec PenningTrap::total_force(int i)
 {
-
+    return this->total_force_external(i) - this->total_force_particles(i);
 }
 
 void PenningTrap::evolve_RK4(double dt)