Ceilidh ... Re^2: BOND DISSOCIATION ENERGY in PCGAMESS

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Re^2: BOND DISSOCIATION ENERGY in PCGAMESS - HELP!!!

On Sat Dec 13 '08 10:11pm, Alex Granovsky wrote
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I did three separate calculations for every species (one for the parent, then each for the radicals formed). Each run was composed of optimization and frequency calculation at the said method and basis set [B3LYP/6-31+G(3d,p,f)]. Frequency calculation was made since I have read that in computing BDE Values correctly, certain corrections was needed particularly those contribution made from the electronic energy, vibration energy etc. From the computed energies plus the corrections needed, the BDE values of the compounds were calculated. I have also considered many factors such as spin contamination and thankfully, they're not that persistent in my calculation. Also, all dissociation reaction are homolytic reactions. I hope this information could be helpful in answering my questions below.

Thank You Very Much,
James Phil

Original Message:

>>I would like to ask about the consistency of using DFT methods in PCGAMESS to calculate Bond Dissociation Energy of Different Molecules. IM currently using B3LYP/6-31+G(3d,p,f) as my chosen method and basis set and I found out it computes the BDE Values of some compounds reasonably only if the involve radical is Hydrogen (H). However, when I tried to compute the BDE Values of compounds whose dissociation reactions involves other radicals aside from hydrogen (e.g CH3, SiH3), there is significant error in the results. There's a difference of about 6-10 kcals/mole with the computed values with the literature values. What could be the reason for this? Im looking at the capability of DFT methods on integrating electron correlation between molecules. Im also considering the size of the radicals formed. Am I at the right path? Please do help.

Sun Dec 14 '08 9:20am

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