diff -Naur lammps-4Dec08/doc/delete_atoms.html lammps-5Dec08/doc/delete_atoms.html
--- lammps-4Dec08/doc/delete_atoms.html	2008-08-21 07:53:06.000000000 -0600
+++ lammps-5Dec08/doc/delete_atoms.html	2008-12-04 09:15:03.000000000 -0700
@@ -15,7 +15,7 @@
 </P>
 <PRE>delete_atoms style args 
 </PRE>
-<UL><LI>style = <I>group</I> or <I>region</I> or <I>overlap</I> 
+<UL><LI>style = <I>group</I> or <I>region</I> or <I>overlap</I> or <I>porosity</I> 
 
 <PRE>  <I>group</I> args = group-ID
   <I>region</I> args = region-ID
@@ -23,7 +23,10 @@
     cutoff = delete one atom from pairs of atoms within the cutoff (distance units)
     group1-ID = one atom in pair must be in this group
     group2-ID = other atom in pair must be in this group
-
+  <I>porosity</I> args = region-ID fraction seed
+    region-ID = region within which to perform deletions
+    fraction = delete this fraction of atoms
+    seed = random number seed (positive integer) 
 </PRE>
 
 </UL>
@@ -32,7 +35,8 @@
 <PRE>delete_atoms group edge
 delete_atoms region sphere
 delete_atoms overlap 0.3 all all
-delete_atoms overlap 0.5 solvent colloid 
+delete_atoms overlap 0.5 solvent colloid
+delete_atoms porosity cube 0.1 
 </PRE>
 <P><B>Description:</B>
 </P>
@@ -61,6 +65,13 @@
 minimum number of atoms will be deleted, or that the same atoms will
 be deleted when running on different numbers of processors.
 </P>
+<P>For style <I>porosity</I> a specified <I>fraction</I> of atoms are deleted
+within the specified region.  For example, if fraction is 0.1, then
+10% of the atoms will be deleted.  The atoms to delete are chosen
+randomly.  There is no guarantee that the exact fraction of atoms will
+be deleted, or that the same atoms will be deleted when running on
+different numbers of processors.
+</P>
 <P>After atoms are deleted, if the system is not molecular (no bonds),
 then atom IDs are re-assigned so that they run from 1 to the number of
 atoms in the system.  This is not done for molecular systems, since it
diff -Naur lammps-4Dec08/doc/delete_atoms.txt lammps-5Dec08/doc/delete_atoms.txt
--- lammps-4Dec08/doc/delete_atoms.txt	2008-08-21 07:53:06.000000000 -0600
+++ lammps-5Dec08/doc/delete_atoms.txt	2008-12-04 09:15:03.000000000 -0700
@@ -12,14 +12,17 @@
 
 delete_atoms style args :pre
 
-style = {group} or {region} or {overlap} :ulb,l
+style = {group} or {region} or {overlap} or {porosity} :ulb,l
   {group} args = group-ID
   {region} args = region-ID
   {overlap} args = cutoff group1-ID group2-ID
     cutoff = delete one atom from pairs of atoms within the cutoff (distance units)
     group1-ID = one atom in pair must be in this group
     group2-ID = other atom in pair must be in this group
-:pre
+  {porosity} args = region-ID fraction seed
+    region-ID = region within which to perform deletions
+    fraction = delete this fraction of atoms
+    seed = random number seed (positive integer) :pre
 :ule
 
 [Examples:]
@@ -27,7 +30,8 @@
 delete_atoms group edge
 delete_atoms region sphere
 delete_atoms overlap 0.3 all all
-delete_atoms overlap 0.5 solvent colloid :pre
+delete_atoms overlap 0.5 solvent colloid
+delete_atoms porosity cube 0.1 :pre
 
 [Description:]
 
@@ -56,6 +60,13 @@
 minimum number of atoms will be deleted, or that the same atoms will
 be deleted when running on different numbers of processors.
 
+For style {porosity} a specified {fraction} of atoms are deleted
+within the specified region.  For example, if fraction is 0.1, then
+10% of the atoms will be deleted.  The atoms to delete are chosen
+randomly.  There is no guarantee that the exact fraction of atoms will
+be deleted, or that the same atoms will be deleted when running on
+different numbers of processors.
+
 After atoms are deleted, if the system is not molecular (no bonds),
 then atom IDs are re-assigned so that they run from 1 to the number of
 atoms in the system.  This is not done for molecular systems, since it
diff -Naur lammps-4Dec08/doc/fix_langevin.html lammps-5Dec08/doc/fix_langevin.html
--- lammps-4Dec08/doc/fix_langevin.html	2008-05-21 16:45:59.000000000 -0600
+++ lammps-5Dec08/doc/fix_langevin.html	2008-12-04 09:15:03.000000000 -0700
@@ -23,7 +23,7 @@
 
 <LI>damp = damping parameter (time units) 
 
-<LI>seed = random # seed to use for white noise (positive integer) 
+<LI>seed = random number seed to use for white noise (positive integer) 
 
 <LI>zero or more keyword/value pairs may be appended 
 
diff -Naur lammps-4Dec08/doc/fix_langevin.txt lammps-5Dec08/doc/fix_langevin.txt
--- lammps-4Dec08/doc/fix_langevin.txt	2008-05-21 16:45:59.000000000 -0600
+++ lammps-5Dec08/doc/fix_langevin.txt	2008-12-04 09:15:03.000000000 -0700
@@ -16,7 +16,7 @@
 langevin = style name of this fix command :l
 Tstart,Tstop = desired temperature at start/end of run (temperature units) :l
 damp = damping parameter (time units) :l
-seed = random # seed to use for white noise (positive integer) :l
+seed = random number seed to use for white noise (positive integer) :l
 
 zero or more keyword/value pairs may be appended :l
 keyword = {scale}
diff -Naur lammps-4Dec08/src/delete_atoms.cpp lammps-5Dec08/src/delete_atoms.cpp
--- lammps-4Dec08/src/delete_atoms.cpp	2008-08-21 07:53:19.000000000 -0600
+++ lammps-5Dec08/src/delete_atoms.cpp	2008-12-04 09:14:49.000000000 -0700
@@ -24,6 +24,7 @@
 #include "neighbor.h"
 #include "neigh_list.h"
 #include "neigh_request.h"
+#include "random_mars.h"
 #include "memory.h"
 #include "error.h"
 
@@ -56,6 +57,7 @@
   if (strcmp(arg[0],"group") == 0) delete_group(narg,arg);
   else if (strcmp(arg[0],"region") == 0) delete_region(narg,arg);
   else if (strcmp(arg[0],"overlap") == 0) delete_overlap(narg,arg);
+  else if (strcmp(arg[0],"porosity") == 0) delete_porosity(narg,arg);
   else error->all("Illegal delete_atoms command");
 
   // delete local atoms flagged in dlist
@@ -301,3 +303,34 @@
     }
   }
 }
+
+/* ----------------------------------------------------------------------
+   create porosity by deleting atoms in a specified region 
+------------------------------------------------------------------------- */
+
+void DeleteAtoms::delete_porosity(int narg, char **arg)
+{
+  if (narg != 4) error->all("Illegal delete_atoms command");
+
+  int iregion = domain->find_region(arg[1]);
+  if (iregion == -1) error->all("Could not find delete_atoms region ID");
+
+  double porosity_fraction = atof(arg[2]);
+  int seed = atoi(arg[3]);
+
+  RanMars *random = new RanMars(lmp,seed + comm->me);
+
+  // allocate and initialize deletion list
+ 
+  int nlocal = atom->nlocal;
+  dlist = (int *) memory->smalloc(nlocal*sizeof(int),"delete_atoms:dlist");
+  for (int i = 0; i < nlocal; i++) dlist[i] = 0;
+
+  double **x = atom->x;
+
+  for (int i = 0; i < nlocal; i++)
+    if (domain->regions[iregion]->match(x[i][0],x[i][1],x[i][2]))
+      if (random->uniform() <= porosity_fraction) dlist[i] = 1;
+}
+
+
diff -Naur lammps-4Dec08/src/delete_atoms.h lammps-5Dec08/src/delete_atoms.h
--- lammps-4Dec08/src/delete_atoms.h	2008-01-24 15:30:58.000000000 -0700
+++ lammps-5Dec08/src/delete_atoms.h	2008-12-04 09:14:49.000000000 -0700
@@ -29,6 +29,7 @@
   void delete_group(int, char **);
   void delete_region(int, char **);
   void delete_overlap(int, char **);
+  void delete_porosity(int, char **);
 };
 
 }