Negative Values in E_DENSITY_CUBE Files

[Brian Day]

I'm trying to generate .cube files of the electron density (probability 
density). I am using the following lines in my input files (also attached):

&PRINT
   &MO_CUBES
      NHOMO -1
      NLUMO -1
   &END MO_CUBES
   &E_DENSITY_CUBE
   &END E_DENSITY_CUBE
&END PRINT

I have noticed in the .cube file for the electron density (also attached), 
there are a number of values which are negative. Is this due to a numerical 
evaluation, or is there another reason for this that anyone is aware of? 
The negative values seem to range from -1e-18 to -1e-14, and the positive 
values seem to range from 1e-18 to 1e-9. Any help would be greatly 
appreciated!

Thanks and Regards,
     Brian

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