From schiffer -8 at 8- h1tw0036.hoechst.com Thu Feb 5 10:51:42 1998 Received: from mailgate.hoechst.com for schiffer _-at-_)h1tw0036.hoechst.com by www.ccl.net (8.8.3/950822.1) id KAA02051; Thu, 5 Feb 1998 10:39:43 -0500 (EST) Received: by mailgate.hoechst.com id QAA25873; Thu, 5 Feb 1998 16:42:43 +0100 Received: from unknown(134.81.76.36) by mailgate id sma025433; Thu Feb 5 16:37:37 1998 Received: from h1tw0024 by h1tw0036 via SMTP (940816.SGI.8.6.9/940406.SGI.AUTO) id QAA15478; Thu, 5 Feb 1998 16:30:56 +0100 Message-ID: <34D9DB08.41C6 -x- at -x- h1tw0036.hoechst.com> Date: Thu, 05 Feb 1998 16:30:16 +0100 From: "Dr. Heinz Schiffer" Organization: Hoechst Research & Technology X-Mailer: Mozilla 2.0 (X11; I; IRIX 5.3 IP22) MIME-Version: 1.0 To: Pang Siu Kwong CC: Computational Chemistry List Subject: Re: CCL:Difference of HOMO of water molecule calculated by HF, MP2, and DFT References: Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit 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!-h1tw0036.hoechst.com 65926 Frankfurt am Main Schiffer -x- at -x- CRT.hoechst.com