diff --git a/BEAMS3D/Sources/beams3d_physics_mod.f90 b/BEAMS3D/Sources/beams3d_physics_mod.f90
index 02d195c3..8550398f 100644
--- a/BEAMS3D/Sources/beams3d_physics_mod.f90
+++ b/BEAMS3D/Sources/beams3d_physics_mod.f90
@@ -387,7 +387,7 @@ SUBROUTINE beams3d_physics_gc(t, q)
             !-----------------------------------------------------------
             dve   = speed*tau_spit_inv*(1-2*te_temp*inv_mymass*e_charge/speed**2.0)
             dvi   = vc3_tauinv/(speed*speed)*(1+ti_temp*inv_mymass*e_charge/speed**2.0)
-            reduction = (dve + dvi)*dt-sigma*zeta
+            reduction = (dve + dvi)*dt+sigma*zeta
             newspeed = speed - reduction
             dve=dve+ddve
             dvi=dvi+ddvi