From chemistry-request ":at:" ccl.net Wed May 28 16:11:16 2003 Received: from srvus010.unitedcatalysts.com (srvus010.unitedcatalysts.com [208.23.162.8] (may be forged)) by server.ccl.net (8.12.8/8.12.8) with SMTP id h4SKBGgC016346 for ; Wed, 28 May 2003 16:11:16 -0400 Received: by lvlxch02.ntdomain.americas.sc-world.com with Internet Mail Service (5.5.2653.19) id ; Wed, 28 May 2003 16:09:12 -0400 Message-ID: <5CF08BBFE6DE97478C1E76E654CAF90801BC0BEC.-at-.lvlxch02.ntdomain.americas.sc-world.com> From: "Shobe, Dave" To: chemistry.-at-.ccl.net Subject: CCL: RE: Orbitals Date: Wed, 28 May 2003 16:09:12 -0400 MIME-Version: 1.0 X-Mailer: Internet Mail Service (5.5.2653.19) Content-Type: multipart/mixed; boundary="----=_NextPartTM-000-67f96a30-3d23-4b14-834c-ca372b14676f" This message is in MIME format. Since your mail reader does not understand this format, some or all of this message may not be legible. ------=_NextPartTM-000-67f96a30-3d23-4b14-834c-ca372b14676f Content-Type: multipart/alternative; boundary="----_=_NextPart_001_01C32555.0155E480" ------_=_NextPart_001_01C32555.0155E480 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable This may only show that my mind is *Really* contaminated by the calculational methods, but I tend to think of the orbitals as the mathematical building blocks out of which the wave function is = constructed, as a linear combination of the determinants thereof. A multi-electron = wave function is just too complicated to comprehend in its entirety. =20 But (I think this is true) if you include the contributions of all determinants made from the entire set of orbitals--i.e. full CI with no approximations--the orbitals are not unique and any set of orbitals = that "spans the vector space," as a mathematician would put it, can be used. --David Shobe S=FCd-Chemie Inc. phone (502) 634-7409 fax (502) 634-7724 email dshobe.-at-.sud-chemieinc.com Don't bother flaming me: I'm behind a firewall. P.S. I wonder if one could define the HOMO at least by reversing = Koopman's theorem and calculating the HOMO from the electron density difference between the molecule and its "vertical" radical cation. And similarly = with the LUMO using the vertical radical anion, if it exists. Or does one necessarily run into Guosheng Wu's paradox of knowing X^2 but not = knowing X? -----Original Message----- From: Alan.Shusterman.-at-.directory.Reed.EDU [mailto:Alan.Shusterman.-at-.directory.Reed.EDU] Sent: Wednesday, May 28, 2003 1:50 PM To: chemistry.-at-.ccl.net Subject: CCL:Orbitals Some of the postings on this subject have cited *correlations* between orbital properties and physical observables as evidence that orbitals = "in some sense" exist. example: If MOs have no physical reality for multielectron species, why = (a) is Koopmans' theorem useful, why (b) do photoelectron spectra match the predictions = of MO energy-level diagrams, and why (c) does the Hueckel 4n+2 rule, which is based on MO diagrams, work? (E. Lewars) example: Orbitals can also, in a sense, be observed experimentally. (M. Johansson) I'm sympathetic to these statements, but I prefer Jens' point of view. I am writing not to cast my vote, but to point out an interesting (to = me, anyway) parallel. I teach organic chemistry every fall semester, and = every year I must convince my students that *resonance contributors* do not = exist even though one can correlate physical observables with the properties = of these contributors. Some may still insist that, "if we routinely think of object X to make = a prediction, then object X exists, at least in our minds". This is an interesting idea, but I don't think that it relates well to "existence" = in the way chemists use this word. I'd like to thank everyone who has contributed to this discussion so = far. It has been interesting. -Alan =3D=3D=3D=3D Alan Shusterman Department of Chemistry Reed College Portland, OR academic.reed.edu/chemistry/alan/ -=3D This is automatically added to each message by mailing script =3D- To send e-mail to subscribers of CCL put the string CCL: on your = Subject: line and send your message to: CHEMISTRY.-at-.ccl.net Send your subscription/unsubscription requests to: = CHEMISTRY-REQUEST.-at-.ccl.net HOME Page: http://www.ccl.net | Jobs Page: http://www.ccl.net/jobs=20 If your mail is bouncing from CCL.NET domain send it to the maintainer: Jan Labanowski, jkl.-at-.ccl.net (read about it on CCL Home Page) -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+= ------_=_NextPart_001_01C32555.0155E480 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable CCL: RE: Orbitals

This may only show that my mind is *Really* = contaminated by the calculational methods, but I tend to think of the = orbitals as the mathematical building blocks out of which the wave = function is constructed, as a linear combination of the determinants = thereof.  A multi-electron wave function is just too complicated = to comprehend in its entirety. 

But (I think this is true) if you include the = contributions of all determinants made from the entire set of = orbitals--i.e. full CI with no approximations--the orbitals are not = unique and any set of orbitals that "spans the vector space," = as a mathematician would put it, can be used.

--David Shobe
S=FCd-Chemie Inc.
phone (502) 634-7409
fax     (502) 634-7724
email  dshobe.-at-.sud-chemieinc.com

Don't bother flaming me: I'm behind a = firewall.

P.S. I wonder if one could define the HOMO at least = by reversing Koopman's theorem and calculating the HOMO from the = electron density difference between the molecule and its = "vertical" radical cation.  And similarly with the LUMO = using the vertical radical anion, if it exists.  Or does one = necessarily run into Guosheng Wu's paradox of knowing X^2  but not = knowing X?



-----Original Message-----
From: Alan.Shusterman.-at-.directory.Reed.EDU
[mailto:Alan.Shusterma= n.-at-.directory.Reed.EDU]
Sent: Wednesday, May 28, 2003 1:50 PM
To: chemistry.-at-.ccl.net
Subject: CCL:Orbitals


Some of the postings on this subject have cited = *correlations* between orbital properties and physical observables as = evidence that orbitals "in some sense" exist.

example: If MOs have no physical reality for = multielectron species, why (a) is Koopmans'
theorem useful,  why (b) do photoelectron = spectra match the predictions of MO
energy-level diagrams, and why (c) does the Hueckel = 4n+2 rule, which is based on
MO diagrams, work? (E. Lewars)

example: Orbitals can also, in a sense, be observed = experimentally. (M. Johansson)

I'm sympathetic to these statements, but I prefer = Jens' point of view.

I am writing not to cast my vote, but to point out an = interesting (to me, anyway) parallel. I teach organic chemistry every = fall semester, and every year I must convince my students that = *resonance contributors* do not exist even though one can correlate = physical observables with the properties of these = contributors.

Some may still insist that, "if we routinely = think of object X to make a prediction, then object X exists, at least = in our minds". This is an interesting idea, but I don't think that = it relates well to "existence" in the way chemists use this = word.

I'd like to thank everyone who has contributed to = this discussion so far. It has been interesting.

-Alan

=3D=3D=3D=3D
Alan Shusterman
Department of Chemistry
Reed College
Portland, OR
academic.reed.edu/chemistry/alan/



-=3D This is automatically added to each message by = mailing script =3D-
To send e-mail to subscribers of CCL put the string = CCL: on your Subject: line
and send your message to:  = CHEMISTRY.-at-.ccl.net

Send your subscription/unsubscription requests to: = CHEMISTRY-REQUEST.-at-.ccl.net
HOME Page: http://www.ccl.net   | Jobs Page: http://www.ccl.net/jobs

If your mail is bouncing from CCL.NET domain send it = to the maintainer:
Jan Labanowski,  jkl.-at-.ccl.net (read about it on = CCL Home Page)
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