When a PhysicalVolume has boundaries with multiple different PhysicalVolumes, you need to define multiple BorderSurfaces. However, the GDML write operation only write the first defined BorderSurface of each PhysicalVolume to the GDML file.

  For example, A PhysicalVolume called physicalVolumeA has boundaries with physicalVolumeB and physicalVolumeC, the code shows as bellow:

   new G4LogicalBorderSurface(“nameAB”, 
              physicalVolumeA, physicalVolumeB, opSurfaceAB);
   new G4LogicalBorderSurface(“nameAC”, 
              physicalVolumeA, physicalVolumeC, opSurfaceAC);

Then Only the BoderSurfaceAB is writed to GDML file, and the BorderSurfaceAC is ignored.

Suggested fix:
  343 const G4LogicalBorderSurface*
  344 G4GDMLWriteStructure::GetBorderSurface(const G4VPhysicalVolume* const pvol)
  345 {
  346   G4LogicalBorderSurface* surf = 0;
  347   G4int nsurf = G4LogicalBorderSurface::GetNumberOfBorderSurfaces();
  348   if (nsurf)
  349   {
  350     const G4LogicalBorderSurfaceTable* btable =
  351           G4LogicalBorderSurface::GetSurfaceTable();
  352     std::vector<G4LogicalBorderSurface*>::const_iterator pos;
  353     for (pos = btable->begin(); pos != btable->end(); pos++)
  354     {
  355       if (pvol == (*pos)->GetVolume1())  // just the first in the couple 
  356       {                                  // is enough
  357         surf = *pos; //break;
  358         BorderSurfaceCache(surf);  //add this code
  359       }
  360     }
  361   }
  362   return surf;
  363 }

  And in G4GDMLWriteStructure::TraverseVolumeTree(),make the following changes：

  546        //BorderSurfaceCache(GetBorderSurface(physvol));
  547        GetBorderSurface(physvol);