code: public void writeGridInPmeshFormat(String outPutFileName) throws IOException {
          BufferedWriter writer = new BufferedWriter(new FileWriter(outPutFileName + ".pmesh"));
          int numberOfGridPoints = grid.length * grid[0].length * grid[0][0].length;
          writer.write(numberOfGridPoints + "\n");
          for (int z = 0; z < grid[0][0].length; z++) {
              for (int y = 0; y < grid[0].length; y++) {
                  for (int x = 0; x < grid.length; x++) {
                      Point3d coords = getCoordinatesFromGridPoint(new Point3d(x, y, z));
                      writer.write(coords.x + "\t" + coords.y + "\t" + coords.z + "\n");
                  }
              }
          }
          writer.close();
      }
docs: Method transforms the grid into pmesh format.