From chemistry-request@ccl.net Fri May 30 11:39:49 2003 Received: from sungoddess.ccs.yorku.ca (sungoddess.ccs.yorku.ca [130.63.236.144]) by server.ccl.net (8.12.8/8.12.8) with ESMTP id h4UFdngC017546 for ; Fri, 30 May 2003 11:39:49 -0400 Received: from mountain.chem.yorku.ca (mountain.chem.yorku.ca [130.63.133.28]) by sungoddess.ccs.yorku.ca (8.12.9/8.12.8) with SMTP id h4UFdj0P010372 for ; Fri, 30 May 2003 11:39:45 -0400 (EDT) Date: Fri, 30 May 2003 11:27:21 -0400 (EDT) From: Rene Fournier - 1999-07-07 X-Sender: renef)at(mountain.chem.yorku.ca To: chemistry)at(ccl.net Subject: Orbitals In-Reply-To: <200305301206.OAA26570)at(host20.lctn.uhp-nancy.fr> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII I agree with Lou Noodleman's point that we have many mathematical models with different degrees of rigor to approach reality without ever quite reaching it (sounds like calculus...). And I do like orbitals, especially Kohn-Sham orbitals, a lot. But I think there's a difference between, say, an approximate electron density -- far from reality because it is inaccurate -- and a set of orbitals -- far from reality because we're not sure exactly what kind of measurements they should be compared to. That being said, theoretical arguments and comparisons to experiments support the view that KS orbital energies are good approximations to relaxed vertical ionization potentials (Chong, Gritsenko, Baerends, J Chem Phys 116 (2002) 1760), that differences of virtual and occupied KS orbital energies are good approximations to excitation energies (A Savin, CJ Umrigar, X Gonze, Chem Phys Lett 288 (1998) 391), that KS orbitals give good approximations to spherically averaged momentum distributions obtained by electron momentum spectroscopy (P Duffy, DP Chong, ME Casida, DR Salahub, Phys Rev A 50 (1994) 4707; S Hamel, P Duffy, ME Casida, DR Salahub Journal of Electron Spectroscopy and Related Phenomena (2002?)). To me, KS orbitals are meaningful, and are useful for quantitative comparisons even though it's not 100% clear what they should be compared with. Whether they are "real" in some particular philosophical system is not as important for me... but it's still interesting! :) -- Rene Fournier. -------------------------------------------------------------------- | Rene Fournier | Office: 303 Petrie | | Chemistry Dpt, York University| Phone: (416) 736 2100 Ext. 30687 | | 4700 Keele Street, Toronto | FAX: (416)-736-5936 | | Ontario, CANADA M3J 1P3 | e-mail: renef)at(yorku.ca | --------------------------------------------------------------------