Subject: Exchange series divergency.

From: Leonardo Demaria <[email-masked]>
Date: Tue, 9 Apr 1996 11:41:42 +22311408 (MET_DST)

Hi everybody, I'm trying to preform CNDO calculations for solids, diamond 
and graphite in particular. I want to use a large unit cell and only the 
gamma point for Brillouin zone integrations.
On Hartree-Fock calculations for solids the exchange energy diverges for
the gamma point. This causes some convergence problems when Brillouin zone
integrations are performed using special points and becomes a serious pro-
blem when one is trying to do computer simulations using large unit cells
and only gamma point sampling. Usually an artificial cut-off for the exchange
series is introduced.
I know about some ways to efficently handle this divergency when doing calcu-
lations with special points (F.Gygi and A.Baldereschi works) but I've not been 
able to find anything different from the cut-off trick for calculations with 
a large unit cell. 
CNDO, being an approximate Hartree-Fock scheme, inherits this exchange series
divergency problems.
Does anyone know how to handle this problem without using a cut-off?
If don't then, does anyone know where to find useful information about how to
choose the cut-off ? I'll summarize the answers.
 			Thank you in advance,

                          Leonardo De Maria                  

Ph.D. student
Universita di Trieste 
Dipartimento di Fisica Teorica 
Strada Costiera 11
Miramare-Grignano
I-34014 - Trieste
tel:     +39 40 224265
fax:            224601  
e-mail : [email-masked] (also [email-masked] can be used)