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Solntsev Pasha
solntsev@univ.kiev.ua

>Please, I am trying to get the total energy, enthalpy, gibbs free energy of my system. I compute the thermochemical properties of simple molecule "H2O" but I am confused because I obtained two thermochemical output as follows:

>  THERMOCHEMISTRY AT T=  298.15 K
>     -------------------------------

> USING IDEAL GAS, RIGID ROTOR, HARMONIC NORMAL MODE APPROXIMATIONS.
> P=  1.01325E+05 PASCAL.
> ALL FREQUENCIES ARE SCALED BY   1.00000
> THE MOMENTS OF INERTIA ARE (IN AMU*BOHR**2)
>         2.07890        4.38535        6.46425
> THE ROTATIONAL SYMMETRY NUMBER IS  1.0
> THE ROTATIONAL CONSTANTS ARE (IN GHZ)
>    867.32830   411.16213   278.93251
> THE HARMONIC ZERO POINT ENERGY IS (SCALED BY   1.000)
>        0.021773 HARTREE/MOLECULE     4778.512983 CM**-1/MOLECULE
>       13.662468 KCAL/MOL               57.163766 KJ/MOL

>               Q               LN Q
> ELEC.     1.00000E+00       0.000000
> TRANS.    3.00431E+06      14.915558
> ROT.      8.68998E+01       4.464756
> VIB.      1.00017E+00       0.000170
> TOT.      2.61118E+08      19.380484

>              E         H         G         CV        CP        S
>           KJ/MOL    KJ/MOL    KJ/MOL   J/MOL-K   J/MOL-K   J/MOL-K
> ELEC.      0.000     0.000     0.000     0.000     0.000     0.000
> TRANS.     3.718     6.197   -36.975    12.472    20.786   144.800
> ROT.       3.718     3.718   -11.068    12.472    12.472    49.593
> VIB.      57.167    57.167    57.163     0.106     0.106     0.014
> TOTAL     64.604    67.083     9.121    25.050    33.364   194.407

>              E         H         G         CV        CP        S
>         KCAL/MOL  KCAL/MOL  KCAL/MOL CAL/MOL-K CAL/MOL-K CAL/MOL-K
> ELEC.      0.000     0.000     0.000     0.000     0.000     0.000
> TRANS.     0.889     1.481    -8.837     2.981     4.968    34.608
> ROT.       0.889     0.889    -2.645     2.981     2.981    11.853
> VIB.      13.663    13.663    13.662     0.025     0.025     0.003
> TOTAL     15.441    16.033     2.180     5.987     7.974    46.464
> ......END OF NORMAL COORDINATE ANALYSIS......

>How do I get the total E, H, G ? I suppose the E should be the FINAL ENERGY which is -75.5859597519 in the output file. What does "Q" and "LN Q" stands for ?

Q is  partition function. You can read any textbook about statistical thermodynamic or, for example, check this link: http://www.gaussian.com/g_whitepap/thermo.htm

Pavel.



>Thanks.