dplot got all zero densities


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Just Got Here
Hi,

I just started to learn NWChem and tried out this example form http://nwchemgit.github.io/index.php/Release66:DPLOT:

start n2
geometry
  n  0 0   0.53879155
  n  0 0  -0.53879155
end
basis;  n library cc-pvdz;end
scf
vectors  output n2.movecs
end
dplot
  TITLE HOMO
  vectors n2.movecs
   LimitXYZ
 -3.0 3.0 10  
 -3.0 3.0 10 
 -3.0  3.0  10
  spin total
  gaussian
  output chargedensity.cube
end
task scf     
task dplot


However the cube file that I got contains only zeros for the grid data. I am running NWChem on a fedora 24 workstation with
mpirun -np 1 nwchem_openmpi nitrogen.nw


 Cube file generated by NWChem
 HOMO
    2   -5.669180   -5.669180   -5.669180
   11    1.133836    0.000000    0.000000
   11    0.000000    1.133836    0.000000
   11    0.000000    0.000000    1.133836
    7    7.000000    0.000000    0.000000    1.018168
    7    7.000000    0.000000    0.000000   -1.018168
  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
...


and this is the stdout:

                                NWChem Input Module
                                -------------------



  Limits (a.u.) specified for the density plot:
  ---------------------------------------------

        From        To      # of spacings
X    -5.66918     5.66918        10
Y    -5.66918     5.66918        10
Z    -5.66918     5.66918        10

  Total number of grid points =                  1331

  1-st set of MOs      : n2.movecs
  Output is written to : chargedensity.cube
  Type of picture      : CHARGE DENSITY
  Format used          : Gaussian9x Cube
  Spin                 : TOTAL   
  The density is computed using all orbitals
  The density is computed on the specified grid
  max element    0.0000000000000000     

  Sum of elements      =                        0.00000
  Integration volume   =                        1.45764
  Integrated Charge    =                        0.00000


Could someone tell me what is going wrong here? Thanks!