Re: CCL:Difference of HOMO of water molecule calculated by HF, MP2, and
DFT
- From: "Dr. Heinz Schiffer" <schiffer { *at *
} h1tw0036.hoechst.com>
- Organization: Hoechst Research & Technology
- Subject: Re: CCL:Difference of HOMO of water molecule calculated by
HF, MP2, and DFT
- Date: Thu, 05 Feb 1998 16:30:16 +0100
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
--
Dr. Heinz Schiffer Phone ++49-69-305-2330
Hoechst Research & Technology Fax ++49-69-305-81162
Scientific Computing, G864 Email schiffer { *at * }
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