diff --git a/gprMax/grid/mpi_grid.py b/gprMax/grid/mpi_grid.py
index 52de80a7..146aba93 100644
--- a/gprMax/grid/mpi_grid.py
+++ b/gprMax/grid/mpi_grid.py
@@ -24,16 +24,13 @@ from gprMax.grid.fdtd_grid import FDTDGrid
 
 
 class MPIGrid(FDTDGrid):
-    xmin: int
-    ymin: int
-    zmin: int
-    xmax: int
-    ymax: int
-    zmax: int
-
-    comm: MPI.Intracomm
-
-    def __init__(self, mpi_tasks_x: int, mpi_tasks_y: int, mpi_tasks_z: int, comm: Optional[MPI.Intracomm] = None):
+    def __init__(
+        self,
+        mpi_tasks_x: int,
+        mpi_tasks_y: int,
+        mpi_tasks_z: int,
+        comm: Optional[MPI.Intracomm] = None,
+    ):
         super().__init__()
 
         if comm is None:
@@ -52,16 +49,25 @@ class MPIGrid(FDTDGrid):
         self.rank = self.comm.rank
         self.size = self.comm.size
 
+        self.xmin = 0
+        self.ymin = 0
+        self.zmin = 0
+        self.xmax = 0
+        self.ymax = 0
+        self.zmax = 0
+
     def initialise_field_arrays(self):
         super().initialise_field_arrays()
 
-        self.local_grid_size_x = self.nx // self.mpi_tasks_x
-        self.local_grid_size_y = self.ny // self.mpi_tasks_y
-        self.local_grid_size_z = self.nz // self.mpi_tasks_z
+        local_grid_size_x = self.nx // self.mpi_tasks_x
+        local_grid_size_y = self.ny // self.mpi_tasks_y
+        local_grid_size_z = self.nz // self.mpi_tasks_z
 
-        self.xmin = (self.rank % self.nx) * self.local_grid_size_x
-        self.ymin = ((self.mpi_tasks_x * self.rank) % self.ny) * self.local_grid_size_y
-        self.zmin = ((self.mpi_tasks_y * self.mpi_tasks_x * self.rank) % self.nz) * self.local_grid_size_z
-        self.xmax = self.xmin + self.local_grid_size_x
-        self.ymax = self.ymin + self.local_grid_size_y
-        self.zmax = self.zmin + self.local_grid_size_z
+        self.xmin = (self.rank % self.nx) * local_grid_size_x
+        self.ymin = ((self.mpi_tasks_x * self.rank) % self.ny) * local_grid_size_y
+        self.zmin = (
+            (self.mpi_tasks_y * self.mpi_tasks_x * self.rank) % self.nz
+        ) * local_grid_size_z
+        self.xmax = self.xmin + local_grid_size_x
+        self.ymax = self.ymin + local_grid_size_y
+        self.zmax = self.zmin + local_grid_size_z