diff --git a/postprocessor.py b/postprocessor.py
index 995f2d1..627359c 100644
--- a/postprocessor.py
+++ b/postprocessor.py
@@ -394,7 +394,7 @@ class PostProcessor(HDF5Container):
 
         self.def_rules()
 
-    def load_data(self, points_src, filename, save):
+    def load_data(self, points_src, filename, save, keys=None):
         """
         Load data from the source file in the memory.
         (Long and memory heavy)
@@ -404,7 +404,9 @@ class PostProcessor(HDF5Container):
             try:
                 node = hdf5.get_node("/data")
                 data = {}
-                for key in node._v_children:
+                if keys is None:
+                    keys = node._v_children
+                for key in keys:
                     data[key] = hdf5.get_node("/data/" + key).read()
             finally:
                 hdf5.close()
@@ -430,12 +432,13 @@ class PostProcessor(HDF5Container):
                     hdf5.close()
         return data
 
-    def load_parts(self):
+    def load_parts(self, keys=None):
         if not self.parts_loaded:
             self.parts = self.load_data(
                 self._part,
                 self.parts_filename,
                 self.pp_params.process.save_parts,
+                keys=keys,
             )
             self.parts_loaded = True
 
@@ -448,7 +451,7 @@ class PostProcessor(HDF5Container):
             del self.parts
             self.parts_loaded = False
 
-    def load_cells(self):
+    def load_cells(self, keys=None):
         """
         Load all cells from the source file in the memory.
         Cells will be accessible trough self.cells
@@ -460,6 +463,7 @@ class PostProcessor(HDF5Container):
                 cells_src,
                 self.cells_filename,
                 self.pp_params.process.save_cells,
+                keys=keys,
             )
             self.cells_loaded = True
 
@@ -1285,12 +1289,20 @@ class PostProcessor(HDF5Container):
         pspec_new.pspec(repo=self.path, iouts=[self.num], outfile=outfile, **kwargs)
         return np.array([self.pspec_filename])
 
-    def _particles(self):
+    def _write_particles(self):
         """Ensure particles are written in the hdf5 file"""
-        self.load_parts()
-        self.unload_parts()
+        if not os.path.exists(self.parts_filename) and not self.parts_loaded:
+            self.load_parts()
+            self.unload_parts()
         return np.array([self.parts_filename])
 
+    def _write_cells(self):
+        """Ensure cells are written in the hdf5 file"""
+        if os.path.exists(self.cells_filename) and not self.cells_loaded:
+            self.load_cells()
+            self.unload_cells()
+        return np.array([self.cells_filename])
+
     def _filaments(self):
 
         datamap_name = self.pp_params.filaments.datamap
@@ -1557,7 +1569,10 @@ class PostProcessor(HDF5Container):
                 },
             ),
             "pspec": Rule(self, self._pspec, "Power spectrum", "/hdf5"),
-            "particles": Rule(self, self._particles, "Particles file", "/hdf5"),
+            "write_particles": Rule(
+                self, self._write_particles, "Particles file", "/hdf5"
+            ),
+            "write_cells": Rule(self, self._write_cells, "Cells file", "/hdf5"),
             "filaments": Rule(
                 self,
                 self._filaments,