Subject: what programs for big periodic systems

From: Stefan Fau <[email-masked]>
Date: Tue, 14 Dec 1999 18:55:58 -0500 (EST)

Hi CCLers,

I'm thinking about a project to calculate small molecules adsorbed to 
defective surfaces of some crystal. From the literature I know that 
correlation is important and that my system is not closed shell, i.e. 
alpha and beta densities are different. So I think that the best method 
is DFT, using LSDA and (probably) some GGA (i.e. gradient corrected) 
functional. The problem is that I'm not familiar with the world of 
periodic calculations, I just know of the existance of two programs in 
that area: CRYSTAL and CASTEP. Since my surface is defective, I need to 
cover a surface area of at least 2x2 unit cells and a depth of 3 cells 
(approx. 18 symmetry unique atoms). So I think I also need analytical 
gradients of atomic positions vs. energy if I am to finish geometry 
optimization in a reasonable time. I already know that CRYSTAL does a 
numerical (gradients based on finite energy differences) optimization, 
but what about CASTEP? The information I found on the web was not 
instructive in this respect.

So the main questions are:
* What programs can optimize systems with with 20 or 45 symmetry 
  independent  atoms?
* What programs can work with LSDA+GGA DFT methods?
* What do these programs cost?
* Do they work nicely or are there some points to keep an eye on?

I'll summarize in about a week to the list.

Thanks in advance,
                   Stefan Fau
______________________________________________________________________
Dr. Stefan Fau                  [email-masked]
Quantum Theory Project
University of Florida
Gainesville FL 32611-8435