diff --git a/src/psc_shock.cxx b/src/psc_shock.cxx
index ad70b0b53..0ccc42478 100644
--- a/src/psc_shock.cxx
+++ b/src/psc_shock.cxx
@@ -25,7 +25,6 @@ using PscConfig = PscConfig1vbecDouble<Dim>;
 
 // ----------------------------------------------------------------------
 
-using BgkMfields = PscConfig::Mfields;
 using MfieldsState = PscConfig::MfieldsState;
 using Mparticles = PscConfig::Mparticles;
 using Balance = PscConfig::Balance;
@@ -73,6 +72,8 @@ double len_z;
 
 int out_interval;
 
+bool mirror_domain;
+
 } // namespace
 
 // ======================================================================
@@ -131,6 +132,8 @@ void setupParameters(int argc, char** argv)
   int n_writes = parsedParams.getOrDefault<int>("n_writes", 100);
   out_interval = psc_params.nmax / n_writes;
 
+  mirror_domain = parsedParams.getOrDefault<bool>("mirror_domain", false);
+
   std::ifstream src(path_to_params, std::ios::binary);
   std::ofstream dst("params_record.txt", std::ios::binary);
   dst << src.rdbuf();
@@ -147,17 +150,19 @@ void setupParameters(int argc, char** argv)
 Grid_t* setupGrid()
 {
   // FIXME add a check to catch mismatch between Dim and n grid points early
-  auto domain =
-    Grid_t::Domain{{nx, ny, nz},          // n grid points
-                   {len_x, len_y, len_z}, // physical lengths
-                   {0, 0, 0},             // location of lower corner
-                   {n_patches_x, n_patches_y, n_patches_z}}; // n patches
-
-  auto bc =
-    psc::grid::BC{{BND_FLD_PERIODIC, BND_FLD_CONDUCTING_WALL, BND_FLD_PERIODIC},
-                  {BND_FLD_PERIODIC, BND_FLD_CONDUCTING_WALL, BND_FLD_PERIODIC},
-                  {BND_PRT_PERIODIC, BND_PRT_REFLECTING, BND_PRT_PERIODIC},
-                  {BND_PRT_PERIODIC, BND_PRT_REFLECTING, BND_PRT_PERIODIC}};
+  int y_mult = mirror_domain ? 2 : 1;
+  auto domain = Grid_t::Domain{
+    {nx, y_mult * ny, nz},          // n grid points
+    {len_x, y_mult * len_y, len_z}, // physical lengths
+    {0, 0, 0},                      // location of lower corner
+    {n_patches_x, y_mult * n_patches_y, n_patches_z}}; // n patches
+
+  auto BND_FLD_Y = mirror_domain ? BND_FLD_PERIODIC : BND_FLD_CONDUCTING_WALL;
+  auto BND_PRT_Y = mirror_domain ? BND_PRT_PERIODIC : BND_PRT_REFLECTING;
+  auto bc = psc::grid::BC{{BND_FLD_PERIODIC, BND_FLD_Y, BND_FLD_PERIODIC},
+                          {BND_FLD_PERIODIC, BND_FLD_Y, BND_FLD_PERIODIC},
+                          {BND_PRT_PERIODIC, BND_PRT_Y, BND_PRT_PERIODIC},
+                          {BND_PRT_PERIODIC, BND_PRT_Y, BND_PRT_PERIODIC}};
 
   auto kinds = Grid_t::Kinds(NR_KINDS);
   kinds[KIND_ELECTRON] = {-1.0, electron_mass, "e"};
@@ -198,10 +203,11 @@ void initializeParticles(Balance& balance, Grid_t*& grid_ptr, Mparticles& mprts)
     double temperature =
       np.kind == KIND_ION ? ion_temperature : electron_temperature;
     np.n = 1.0;
+    double vy_coef = mirror_domain && crd[1] > len_y ? -1 : 1;
     np.p = setup_particles.createMaxwellian(
       {np.kind,
        np.n,
-       {v_upstream_x, v_upstream_y, v_upstream_z},
+       {v_upstream_x, vy_coef * v_upstream_y, v_upstream_z},
        {temperature, temperature, temperature},
        np.tag});
   };
@@ -226,10 +232,11 @@ void fillGhosts(MF& mfld, int compBegin, int compEnd)
 void initializeFields(MfieldsState& mflds)
 {
   setupFields(mflds, [&](int component, double coords[3]) {
+    double e_coef = mirror_domain && coords[1] > len_y ? -1 : 1;
     switch (component) {
-      case EX: return e_x;
-      case EY: return e_y;
-      case EZ: return e_z;
+      case EX: return e_coef * e_x;
+      case EY: return e_coef * e_y;
+      case EZ: return e_coef * e_z;
       case HX: return b_x;
       case HY: return b_y;
       case HZ: return b_z;
@@ -257,9 +264,6 @@ static void run(int argc, char** argv)
   auto& grid = *grid_ptr;
   MfieldsState mflds{grid};
   Mparticles mprts{grid};
-  BgkMfields phi{grid, 1, mflds.ibn()};
-  BgkMfields gradPhi{grid, 3, mflds.ibn()};
-  BgkMfields divGradPhi{grid, 1, mflds.ibn()};
 
   // ----------------------------------------------------------------------
   // Set up various objects needed to run this case
@@ -279,8 +283,7 @@ static void run(int argc, char** argv)
   // -- Checks
   ChecksParams checks_params{};
   checks_params.gauss.check_interval = out_interval;
-  // checks_params.gauss.dump_always = true;
-  checks_params.gauss.err_threshold = 1e-5;
+  checks_params.continuity.check_interval = out_interval;
 
   Checks checks{grid, MPI_COMM_WORLD, checks_params};