AUX[([n][,][COMP][,][PRECISION=m][,][MOS=n][,][XP][,][XS][,][XW])]

When AUX is used then auxiliary information is output to a file called <file>.aux.  When AUX(n) is used, output for the changing geometry is re-directed to output channel n, n is limited to the range 0 - 99. Thus AUX(6) would re-direct the changing geometry to standard output unit 6. Useful values are n = 0 and n = 6. Avoid using units 2 to 4, and 9 to 30, as these are used by MOPAC. The <file>.aux  files are intended for use by Graphical User Interfaces (GUI's), but the files can be converted using a simple Windows utility, Read_AUX, into ASCII tables for use by programs such as EXCEL.

AUX is supported by Avogadro, Gabedit, and Molden.

When COMP is present, all large blocks of data are printed in compressed form. 
To increase precision by m digits (m limited to the range 1 to 9), use PRECISION=m.
By default, only the 10 highest occupied  and 10 lowest unoccupied M.O.s are printed, unless LARGE is present, in which case all M.O.s are printed To change the default, use MOS=n, where n is the number of occupied and virtual M.O.s to be printed.  If n is greater than the number of M.O.s, it will be re-set to all the M.O.s, so an alternative way to print all the M.O.s is to use MOS=99999. To prevent printing of M.O.'s, use MOS=-99999.
The commas are optional.
If the re-direction unit "n" is used together with any of COMP, PRECISION, or MOS, then "n" must come first, followed by the other options.

For large systems, the AUX file might still be very large, even if COMP is used.  To reduce the size of the AUX file, use one or more of the following options:

If XP is present, the density matrix will not be printed.
If XS is present, the overlap matrix will not be printed.
If XW is present, the M.O.s will not be printed.  This is an alternative to MOS=-99999.

Only data considered to be important to GUI programs will be included in the .aux file.  A consequence of this is that some data might be missing.  If such data can be identified, and a message sent to MrMOPAC@ATT.net, then the data will be added to the .aux file.

The auxiliary information is a very compact form of the results of a calculation in a primitive mark-up language. This file is intended for use by other programs. An example of the auxiliary file for water is given below.  No explanation is necessary: the file is intended to be self-descriptive.
  START OF MOPAC FILE
 ####################################
 #                                  #
 #       Start of Input data        #
 #                                  #
 ####################################
 MOPAC_VERSION=MOPAC2007.7.150W
 DATE="Mon Jun  4 12:15:37 2007"
 METHOD=PM6
 TITLE=" Water"
 KEYWORDS=" SYMMETRY"
 ATOM_EL[0003]=
  O  H  H
 ATOM_X:ANGSTROMS[0009]=
    0.0000    0.0000    0.0000
    0.6018    0.7647    0.0000
    0.6018   -0.7647    0.0000
 AO_ATOMINDEX[0006]=
  1  1  1  1  2  3
 ATOM_SYMTYPE[0006]=
  S PX PY PZ  S  S
 AO_ZETA[0006]=
  5.4218  2.2710  2.2710  2.2710  1.2686  1.2686
 ATOM_PQN[0006]=
 2 2 2 2 1 1
 NUM_ELECTRONS=0008
 EMPIRICAL_FORMULA="H2 O"
 ####################################
 #                                  #
 #      Geometry optimization       #
 #                                  #
 ####################################
 HEAT_OF_FORM_UPDATED:KCAL/MOL=-0.540108D+02
 GRADIENT_UPDATED:KCAL/MOL/ANG=+0.210682D+02
 ATOM_X_UPDATED:ANGSTROMS[0009]=
    0.0000    0.0000    0.0000
    0.6018    0.7647    0.0000
    0.6018   -0.7647    0.0000
 HEAT_OF_FORM_UPDATED:KCAL/MOL=-0.542600D+02
 GRADIENT_UPDATED:KCAL/MOL/ANG=+0.893385D+01
 ATOM_X_UPDATED:ANGSTROMS[0009]=
    0.0000    0.0000    0.0000
    0.5693    0.7558    0.0000
    0.5693   -0.7558    0.0000
 HEAT_OF_FORM_UPDATED:KCAL/MOL=-0.543026D+02
 GRADIENT_UPDATED:KCAL/MOL/ANG=+0.225094D+01
 ATOM_X_UPDATED:ANGSTROMS[0009]=
    0.0000    0.0000    0.0000
    0.5641    0.7630    0.0000
    0.5641   -0.7630    0.0000
 HEAT_OF_FORM_UPDATED:KCAL/MOL=-0.543063D+02
 GRADIENT_UPDATED:KCAL/MOL/ANG=+0.144932D+00
 ATOM_X_UPDATED:ANGSTROMS[0009]=
    0.0000    0.0000    0.0000
    0.5610    0.7655    0.0000
    0.5610   -0.7655    0.0000

 ####################################
 #                                  #
 #        Final SCF results         #
 #                                  #
 ####################################
 HEAT_OF_FORMATION:KCAL/MOL=-0.543063D+02
 ENERGY_ELECTRONIC:EV=-0.458386D+03
 ENERGY_NUCLEAR:EV=+0.139308D+03
 POINT_GROUP=C2v
 DIPOLE:DEBYE=+0.206870D+01
 AREA:SQUARE ANGSTROMS=+0.424512D+02
 VOLUME:CUBIC ANGSTROMS=+0.251444D+02
 ATOM_X_OPT:ANGSTROMS[0009]=
    0.0000    0.0000    0.0000
    0.5610    0.7655    0.0000
    0.5610   -0.7655    0.0000
 ATOM_CHARGES[0003]=
 -0.61867 +0.30933 +0.30933
 OVERLAP_MATRIX[000021]=
 #  Lower half triangle only
   1.0000   0.0000   1.0000   0.0000   0.0000   1.0000   0.0000   0.0000   0.0000   1.0000
   0.1618   0.2330   0.3180   0.0000   1.0000   0.1618   0.2330  -0.3180   0.0000   0.2333
   1.0000
 EIGENVECTORS[000036]=
  -0.8288  -0.2451   0.0000   0.0000  -0.3556  -0.3556   0.0000   0.0000   0.8129   0.0000
   0.4118  -0.4118   0.4394  -0.8417   0.0000   0.0000  -0.2220  -0.2220   0.0000   0.0000
   0.0000   1.0000   0.0000   0.0000   0.3463   0.4812   0.0000   0.0000  -0.5694  -0.5694
   0.0000   0.0000   0.5824   0.0000  -0.5748   0.5748
 TOTAL_DENSITY_MATRIX[000021]=
 #  Lower half triangle only
   1.7601  -0.3333   1.5370   0.0000   0.0000   1.3216   0.0000   0.0000   0.0000   2.0000
   0.3944   0.5480   0.6695   0.0000   0.6907   0.3944   0.5480  -0.6695   0.0000   0.0123
   0.6907
 M.O.SYMMETRY_LABELS[0006]=
  1a1     1b2     2a1     1b1     3a1     2b2 
 EIGENVALUES[0006]=
  -30.402  -18.983  -14.111  -11.905    4.067    6.038
 MOLECULAR_ORBITAL_OCCUPANCIES[00006]=
 2.0000 2.0000 2.0000 2.0000 0.0000 0.0000

When COMP is present, the overlap and density matrices, and eigenvectors, are printed in compressed form.
Compare the previous uncompressed data with the following compressed data to see how the compression is done.

 OVERLAP_INDICES[013]=
     1     3     6    10    15    21    18    13    20    17    12    16    11
 OVERLAP_COEFFICIENTS[013]=
   1.0000   1.0000   1.0000   1.0000   1.0000   1.0000  -0.3180   0.3180   0.2336   0.2332
   0.2332   0.1619   0.1618
 SET_OF_MOS=       1       6
 MO_INDICES[004]=
     1     6     5     2
 MO_COEFFICIENTS[004]=
   0.8287   0.3558   0.3557   0.2452
 MO_INDICES[003]=
     3     5     6
 MO_COEFFICIENTS[003]=
   0.8129   0.4118  -0.4118
 MO_INDICES[004]=
     2     1     5     6
 MO_COEFFICIENTS[004]=
   0.8416  -0.4396   0.2219   0.2219
 MO_INDICES[001]=
     4
 MO_COEFFICIENTS[001]=
   1.0000
 MO_INDICES[004]=
     5     6     2     1
 MO_COEFFICIENTS[004]=
   0.5694   0.5693  -0.4812  -0.3465
 MO_INDICES[003]=
     3     6     5
 MO_COEFFICIENTS[003]=
   0.5824   0.5749  -0.5748
 DENSITY_MATRIX_INDICES[014]=
    10     1     3     6    21    15    13    18    12    17    16    11     2    20
 DENSITY_MATRIX_COEFFICIENTS[014]=
   2.0000   1.7599   1.5369   1.3217   0.6908   0.6908   0.6695  -0.6695   0.5480   0.5480
   0.3945   0.3945  -0.3334   0.0124
When COMP and MOS are present, a user-defined set of M.O.s is printed.  The default is the 10 highest M.O.s and 10 lowest virtual M.O.s.
If LARGE is present, all the M.O.s are printed.