diff --git a/run_openmm.py b/run_openmm.py
index a870791..80f50ba 100644
--- a/run_openmm.py
+++ b/run_openmm.py
@@ -24,6 +24,13 @@
 # Add a Monte Carlo barostat to the system for pressure control
 system.addForce(mm.MonteCarloBarostat(1*unit.atmospheres, 300*unit.kelvin, 25))
 
+custom_bond = mm.CustomBondForce("0.5*k*(r-r0)^2")
+custom_bond.addGlobalParameter("k", 100)
+custom_bond.addGlobalParameter("r0", 1)
+custom_bond.addBond(2, 275)
+
+system.addForce(custom_bond)
+
 # Use the CPU platform
 platform = mm.Platform.getPlatformByName('CPU')