CCL: G3B3 Atomization Energy Methane

[John Bushnell <bushnell=-=chem.ucsb.edu>]

 Sent to CCL by: John Bushnell [bushnell**chem.ucsb.edu]
 I'm not at all familiar with the "G3B3" method, but right off hand
 I would think that this is very good agreement with experiment.
 You show an error of 13 kJ/mol at zero K.  But the calculation involves
 the difference of some very large numbers.  The difference of only
 3 kJ/mol at 298 K seems fortuitously small in fact.  If enthalpies
 of formation could be routinely calculated to this accuracy, we
 wouldn't have to do so many experiments. :-)
   Just my offhand impression...
           - John
 On Mon, 20 Feb 2006, Roger Kevin Robinson r.robinson .. imperial.ac.uk wrote:
>  Sent to CCL by: Roger Kevin Robinson [r.robinson,,imperial.ac.uk]
>  Hi,
>     I've asked about this before but i still seem to be having some
>  trouble. Im just using Methane as an example.
>  Using G3B3 methods.
>  At 298K I get
>  Name    G3-Energy(G3B3)    ZPE
>  C          -37.778738
>  H          -0.499671
>  CH4     -40.455401             0.043410
>  Using this values to calculate Atomization Energy
>  = (-37.778738 + 4* -0.499671) - -40.455401  -   0.043410= 0.634569 =
>  1666.06091 kJ/mol
>  this fits in well with an experimental value of 1663.3
>  to calculate Enthalpy of formation you need the atomization Energy at 0K
>  as far as im aware.
>  Right at 0K
>  Name    G3(0K) - (G3B3)      ZPE
>  C         -37.780154
>  H         -0.501087
>  CH4     -40.458277             0.043410
>  = ( -37.780154 +4 * -0.499671) - -40.455401  -   0.043410 = 0.630365 =
>  1655.02331.
>  But the experimental value is 1642.27
>  Does any one know where im making the mistake ?
>  Thanks Roger