diff --git a/src/USER-SMD/smd_material_models.cpp b/src/USER-SMD/smd_material_models.cpp
index 228d1c709a77870ecb752a31580cfa6c571d8ff2..f1288ae5cad31aa8c69b8f9b1df46f6a88e1de5b 100644
--- a/src/USER-SMD/smd_material_models.cpp
+++ b/src/USER-SMD/smd_material_models.cpp
@@ -75,7 +75,7 @@ void ShockEOS(double rho, double rho0, double e, double e0, double c0, double S,
 	double mu = rho / rho0 - 1.0;
 	double pH = rho0 * square(c0) * mu * (1.0 + mu) / square(1.0 - (S - 1.0) * mu);
 
-	pFinal = (pH + rho * Gamma * (e - e0));
+	pFinal = (-pH + rho * Gamma * (e - e0));
 
 	//printf("shock EOS: rho = %g, rho0 = %g, Gamma=%f, c0=%f, S=%f, e=%f, e0=%f\n", rho, rho0, Gamma, c0, S, e, e0);
 	//printf("pFinal = %f\n", pFinal);