AM1 under GAUSSIAN ???

 Dear Netters,
 we have recently carried out a series of calculations
 using MOPAC/AM1.  When we asked academic partners to do some ab initio
 calculations on the same set of compounds we were surprised to find, that
 there
 seems to be an inconcistency between GAUSSIAN/AM1 and MOPAC/AM1,
 concerning the heat capacity Cp. Does anyone know, if this is:
  -> a well known phenomenon/problem/bug
  -> a consequence of a different implementation (deliberate) or
  -> one of the great mysteries of life?
 For your orientation I have appended parts of the MOPAC and GAUSSIAN output
 of calcs for the CO2 molecule.
 Best  Regards
 Lutz Ackermann
 _________________________________________________________
 Dr. Lutz Ackermann
 Service Chimie Physique, Groupment de Recherches de Lacq
 elf atochem
 R.N 117, B.P. 34, F-64170 Lacq (France)
 Phone: (0033) 5 59 92 67 66   FAX: (0033) 5 59 92 67 65
 e-mail: lutz.ackermann |-at-| CALDCRD.elf-atochem.fr or lutz |-at-| ri.ac.uk
 or
 phone/FAX: (0033) 5 59 27 65 43
 lutz.ackermann |-at-| wanadoo.fr
 ___________________________________________________________
 CO2 (AM1)
 MOPAC 6.0:
  ----------
                     CALCULATED THERMODYNAMIC PROPERTIES
                                           *
    TEMP. (K)   PARTITION FUNCTION   H.O.F.    ENTHALPY   HEAT CAPACITY
  ENTROPY
                                     KCAL/MOL   CAL/MOLE    CAL/K/MOL
   CAL/K/MOL
     273  VIB.          1.139                   203.06527    2.22755
    1.00258
            ROT.     255.                        542.522      1.987
     12.997
              INT.     290.                        745.587      4.215
     14.000
            TRA.         0.248E+27                   1356.305      4.968
     36.817
            TOT.                       -80.062  2101.8922     9.1830
    50.8170
     298  VIB.         1.181                   261.82494    2.46932
    1.20837
          ROT.     278.                        592.203      1.987     13.172
          INT.     328.                        854.028      4.457     14.380
          TRA.    0.282E+27                   1480.509      4.968     37.252
          TOT.                       -79.829  2334.5371     9.4247    51.6320
 GAUSSIAN:
  --------
  ...
 Energy=   -0.127281006429 NIter=   9.
  ...
                     E (Thermal)             CV                S
                       KCAL/MOL        CAL/MOL-KELVIN    CAL/MOL-KELVIN
  TOTAL                    9.040              7.411             51.623
  ELECTRONIC               0.000              0.000              0.000
  TRANSLATIONAL            0.889              2.981             37.270
  ROTATIONAL               0.592              1.987             13.171
  VIBRATIONAL              7.559              2.443              1.182
  VIBRATION  1             0.886              1.182              0.586
  VIBRATION  2             0.887              1.179              0.583
  ...
 Transforming the Energy given by GAUSSIAN from Hartree to kcal/mol, one
 gets -79.8653 kcal/mol, which is close to the Heat of Formation calculated
 from MOPAC (at 298K), -79.829 kcal/mol. The entropy is also very similar
 between
 the two programs, but the heat capacity differs a lot ( 7.411 CAL/MOL-KELVIN
 in
 GAUSSIAN vs. 9.4247 CAL/K/MOL in MOPAC!!???