 This example shows how to obtain a simulated STM image of the AlAs(110)
 surface. The image is obtained by  integrating the density of states
 from the bias potential to the fermi energy, as proposed by Tersoff and
 Hamann [PRB 31, 805 (1985)].
 The local density of states contributing to STS is then evaluated as a
 function of energy, with spatial integration in small volumes above
 different surface atoms.

1) make a self-consistent calculation for the relaxed AlAs (110) surface
   by using a slab supercell.
   (input=AlAs110re.scf.in, ouput=AlAs110re.scf.out)

2) make a non-self-consistent calculation for the same cell with a larger
   number of k points
   (input=AlAs110re.nonscf.in, output=AlAs110re.nonscf.out)

3) call the postprocessing executable 'pp.x' with the instruction
   'plot_num= 5' to generate the STM simulated image with bias 
   potential specified by 'sample_bias=-0.0735'. Intermediate data 
   is collected in file 'AlAsresm-1.0'. A 2-dimensional slice of 
   the image is then extracted and written to 'AlAs110-1.0'. 
   (input=AlAs110.pp_stm-.in, and output=AlAs110.pp_stm-.out)

4) same as point 3) but with positive bias 'sample_bias=0.0735'.
   Output files: 'AlAsresm+1.0' and 'AlAs110+1.0'. 
   (input=AlAs110.pp_stm+.in, and output=AlAs110.pp_stm+.out)

5-6) produce a postscript file of the simulated STM images by running the
    executable 'plotrho.x'. This generate the PostScript files
    'AlAs110-1.0eV.ps' and 'AlAs110+1.0eV.ps'.
   (input=AlAs110.plotrho-.in, output=AlAs110.plotrho-.out)
   (input=AlAs110.plotrho+.in, output=AlAs110.plotrho+.out)

7) computes the local density of states, as function of energy, integrated
   in volumes outside the AlAs(110) surface, for small volumes on top of
   the surface Al and As atoms and at various distances from the surface.
   Output files: 'AlAs110.ldos_boxes' which contains the LDOS(E)
   and 'AlAs110.box#<N>.xsf' with XCrySDen structure files.
   (input=AlAs110.box.projwfc.in, output=AlAs110.box.projwfc.out)

8) computes the projected density of states (see example08), for comparison.
   (input=AlAs110.projwfc.in, output=AlAs110.projwfc.out)

9) plot the LDOS and PDOS by using 'gnuplot', if available. This generate
   the PostScript file 'AlAs110.box.projwfc.ps'
