--- trunk/OOPSE/libmdtools/Thermo.cpp	2003/04/04 19:47:19	458
+++ trunk/OOPSE/libmdtools/Thermo.cpp	2003/04/07 16:56:38	468
@@ -139,7 +139,39 @@ double Thermo::getPressure(){
   // routine derived via viral theorem description in:
   // Paci, E. and Marchi, M. J.Phys.Chem. 1996, 100, 4314-4322
 
-  return 0.0;
+  const double convert = 4.184e-4;
+  double mtot;
+  double vcom[3];
+  double p_local, p_sum, p_mol, virial;
+  double theBox[3];
+  double* tau;
+  int i, nMols;
+  Molecule* molecules;
+
+  nMols = entry_plug->n_mol;
+  molecules = entry_plug->molecules;
+  tau = entry_plug->tau;
+
+  // use velocities of molecular centers of mass and molecular masses:
+  p_local = 0.0;
+  for (i=0; i < nMols; i++) {
+    molecules[i].getCOMvel(mtot, vcom);
+    p_local += mtot* (vcom[0]*vcom[0] + vcom[1]*vcom[1] + vcom[2]*vcom[2]);
+  }
+
+  // Get total for entire system from MPI.
+#ifdef IS_MPI
+  MPI_Allreduce(&p_local,&p_sum,1,MPI_DOUBLE, MPI_SUM, MPI_COMM_WORLD);
+#else
+  p_sum = p_local; 
+#endif // is_mpi
+
+  virial = tau[0] + tau[4] + tau[8];
+  entry_plug->getBox(theBox);
+
+  p_mol = (p_sum - virial*convert) / (3.0 * theBox[0] * theBox[1]* theBox[2]);
+  
+  return p_mol;
 }
 
 void Thermo::velocitize() {