[Pw_forum] Format of .pp postprocessing file

[Johannes Moeller]

Dear All,

I am interested in calculating the electric field inside a solid (in zero applied external field). For this purpose I would like to take a .pp file of the ionic+Hartree potential and differentiate it. I couldn't find any information on the internet on the exact structure of a .pp file. Please could this be clarified? 

As an example, the header of my .pp file is as follows: 

     162     162     162     162     162     162      96       2
     1       20.64336828      0.00000000      0.00000000      0.00000000      0.00000000      0.00000000
     6476.6829010408        4.0000000000      150.0000000000    11
   1   Ca   10.00
   2   F     7.00
   1       0.000000000    0.000000000    0.000000000    1
   2       0.000000000    0.000000000    0.500000000    1
   ...
then follow the positions of the ions in the cell and then the density data

 -8.358584553E+00 -8.262687629E+00 -7.991916719E+00 -7.550254499E+00 -6.949854525E+00
 -6.221200847E+00 -5.400129088E+00 -4.537549114E+00 -3.695385287E+00 -2.927873052E+00
 -2.232393469E+00 -1.599364989E+00 -1.053855115E+00 -6.077427805E-01 -2.928590662E-01
...

I can identify some of the numbers:

I'm guessing the grid is 162x162x162 (why is this repeated?) 96 the number of atoms per cell, 2 the number of atomic types
1 is probably the ibrav index then follow celldm(1) to celldm(6) 
not sure about the next 2 numbers, 150 is the wfc cutoff 11 is the identifier for Hartree+ionic potential

I'm also guessing that the innermost index runs fastest, i.e. if pp.x was written in C it would do something like this

 for (ix=0;ix<NX;ix++) {
      for (iy=0;iy<NY;iy++) {
         for (iz=0;iz<NZ;iz++) {
            printf("%g ",data[ix][iy][iz]);
            if ((ix+iy+iz) % 5 == 4)
               printf("\n");
         }
      }
   }

In any case I think it would be good to have a complete description available somewhere in the Quantum Espresso documentation (unless I missed it in which case please point me to it!). 

Thanks,
Johannes

-------------------------------
Johannes Moeller 
Department of Physics
University of Oxford