Re: CCL:Difference of HOMO of water molecule calculated by HF, MP2, and DFT



 Pang Siu Kwong wrote:
 >
 > Hello!  I have used the 4-31G for calculation of HOMO of water.  HF,
 > MP4(SDQ) and DFT(BP86) are tried.  I found that the results of HF and
 > MP4(SDQ) are near the same.  However, the result of DFT is around half of
 > those of HF and MP4(SDQ).  Why?
 > Thank you for your kindly attention.
 >
 > Patrick Pang
 Hi Patrick,
 HF and MPn must be the same ! Molecular orbitals does only exist
 in mean field theories like HF. Within HF the negative orbital energy
 of the HOMO is according to Koopman's theorem approximately equal
 to the ionization potential (IP) of the molecule ( within 10 % of the
 experimental value if approbiate basis sets are used ). DFT methods
 underestimate IPs by a large amount if calculated as negative
 HOMO energies. See e.g.
 U. Salzner et al., J. Comput. Chem. 18(15) (1997) 1943-1953.
 Ciao,
 Heinz
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