Subject: core ionization potentials

From: Luca Pedocchi <[email-masked]>
Date: Fri, 24 Jan 1997 15:01:59 +0100

Dear CCL members, here is my question. I'm new to gaussian code in
general and gaussian94 in particular: is there a way to ionize core
orbitals, let say for instance C 1s from CO? I tried the option
guess=alter but it does not seem to work. Any hint?
Best regards
                  Luca
-- 
Dr. Luca Pedocchi
Laboratorio di Chimica Fisica delle Interfasi
via Cavour, 82 - 50129 Firenze - ITALY

http://lcfs.chim.unifi.it/solid/pedocchi
e-mail [email-masked]

phone +39-55-2757793
fax   +39-55-219802

Non esistono uomini cattivi, se sono cucinati bene
                                            (Stefano Benni)

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From: [email-masked]
Date: Fri, 24 Jan 1997 15:32:38 -0500 (EST)

> guess=alter but it does not seem to work. Any hint?
My guess is that this goal, while quite interesting, is probably hopeless.
Most of the current crop of electronic structure methods (HF, DFT,
semi-empirical...) are based on ground state theories.  There are some
more or less awful methods for low energy excited states (CIS, ZINDO...),
but I would think that a carbon 1s hole in CO will have an energy so high
into the ionization continuum that none of these methods will do any good.
(After all, the Auger spectrum of CO is known.....)

Steve Williams
ASU
Boone, NC 28608
USA

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From: Stephen Urquhart <[email-masked]>
Date: Fri, 24 Jan 1997 19:04:00 -0500 (EST)

To calculate core ionization potentials by SCF methods, one needs to do 
two calculations: a ground state calculation, and a calculation with an 
exclicit core hole. The difference in totoal energy between the two 
calculation is the ionization potential. A calculation with the expilcit 
core hole is required to account for the drastic reorganization of the 
electronic structure upon creation of the core hole - a flexible basis 
set on the core excited atom site helps too. This is the delta(SCF) method.
The results can be quite excellent. For example, for the N 1s ionization 
of NO is calculated by this method to occur at 409.22 eV, and experimentally 
occurs at 410.3 eV (Kosugi et a, J. Chem. Phys 97, 1992, 8842). The 
Koopman's theorm result is probably above 430 eV!

To create the explcit core hole in an SCF calculation, the Static exchange 
hamiltonian is used (See Hunt and Goddard, Chem. Phys. Lett, 3(6) 1968, 
414). This is also called "Improved Virtual Orbitals". I don't know if G94 
has this method, but the program DALTON apparently does. The URL for 
their web page is http://www.uio.no/~kenneth/dalton/dalt.html  I havn't 
used this code so I don't know for sure.

I don't have any good references for delta SCF results at my finger tips, 
but that should be a good keyword for chem abstracts. Delano Chong uses 
density funcitonal theory for core binding energies (J. Chem Phys, 100, 
1996, 19455), also with good results.

Best of luck!
Stephen Urquhart