From owner-chemistry@ccl.net Mon Apr 16 14:53:01 2018
From: "Susi Lehtola susi.lehtola{}alumni.helsinki.fi" <owner-chemistry-*-server.ccl.net>
To: CCL
Subject: CCL: Choice of multiplicity for NiO
Message-Id: <-53238-180416122422-13085-XXYpgHY34NAHAtitNRpfkA-*-server.ccl.net>
X-Original-From: Susi Lehtola <susi.lehtola::alumni.helsinki.fi>
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Date: Mon, 16 Apr 2018 19:24:12 +0300
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Sent to CCL by: Susi Lehtola [susi.lehtola||alumni.helsinki.fi]
On 04/15/2018 10:54 AM, Grigoriy Zhurko reg_zhurko..chemcraftprog.com wrote:
> Sent to CCL by: Grigoriy Zhurko [reg_zhurko%a%chemcraftprog.com] 
> Hello, I compute Ni4O4, Ni9O4 clusters and their complexes with
> carbon monoxide (me research is devoted to CO absorption on NiO).
> These systems can be computed in different multiplicities. Two
> approaches can be used for choosing the multiplicities: 1) Compute
> each multiplicity and find the one with minimal energy; 2) In the
> work of Thomas Bredow, only the multiplicity 19 was chosen for Ni9O9.
> He writes in his paper:
The first option works in simple cases, but for transition metal 
compounds in general - and transition metal oxides appear to be 
especially nasty - getting the right energy ordering for the different 
spin states may be highly nontrivial. Usually DFT gets it wrong, i.e. 
you get different orderings with different functionals, in which case 
the results shouldn't be trusted.

But, ab initio methods may not work very well either. In addition to 
significant effects due to strong correlation (for which you need 
something like CAS-SCF or DMRG-SCF), dynamic correlation can also play a 
significant part in the energy splittings between the spin states: the 
true spin state ordering is due to a delicate balance between static and 
dynamic correlation.

However, your system, Ni9O9 would appear to be well outside the 
computational tractability of reliable ab initio methods. So, I guess 
most people would just go with option 1: pick a functional that (happens 
to) give the right spin state ordering for a closely similar system, and 
then pick the spin state that has the lowest energy.
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Mr. Susi Lehtola, PhD             Junior Fellow, Adjunct Professor
susi.lehtola]![alumni.helsinki.fi   University of Helsinki
http://www.helsinki.fi/~jzlehtol  Finland
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Susi Lehtola, dosentti, FT        tutkijatohtori
susi.lehtola]![alumni.helsinki.fi   Helsingin yliopisto
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