CCL: negative values in Mulliken spin distribution

Hello,

There seems to be an explanation to negative spin densities in radicals, e.g., see this book chapter

digital.csic.es/bitstream/10261/76135/1/Spin%20Density.pdf

On p. 170, the author says, "The negative spin density at the carbon atom is due to a polarisation effect produced by the SOMO orbital on some of the doubly occupied orbitals. ...."


On Wed, Jan 22, 2014 at 12:20 PM, Martin Rahm martin.rahm- -gmail.com <owner-chemistry|,|ccl.net> wrote:
Hi, 

To my knowledge negative atomic orbital occupations in Mulliken population analysis can sometimes occur due to MPAs direct partitioning of CMOs into non-orthogonalized atomic orbitals. Hoffman et al. have rationalized this effect for Huckel and limited basis set SCF calculations, as being a natural consequence of the wave function normalization in certain cases (J. chem. Phys. 68, 5498 and J. Am. Chem. Soc. 100, 3686 ). 

Best, 
Martin 


On Wed, Jan 22, 2014 at 9:57 AM, Raghu R raghu.rama.chem[]gmail.com <owner-chemistry.:.ccl.net> wrote:
Hello,

Negative spin density  is encountered in approximate theories like Hückel MO theory. For a system with one unpaired electron, the sum of spin-densities on all atomic sites add up to 1, but the sum of
their absolute values
is > 1, implying that some sites have negative densities of this odd electron.

See for example,
"Free radicals with large negative spin densities",
M. S. Davis , K. Morokuma , R. W. Kreilick,
J. Am. Chem. Soc., 1972, 94 (16), pp 5588–5592

ps. I do not think that negative density has anything to do with Mulliken's population, in particular.

Best,
Raghu



On Wed, Jan 22, 2014 at 7:36 AM, Haya Kornweitz hayak]|[ariel.ac.il <owner-chemistry%a%ccl.net> wrote:

Sent to CCL by: "Haya  Kornweitz" [hayak===ariel.ac.il]
Can someone explain, or give a reference, the meaning of negative values in
Mulliken spin distribution? thanks a lot.



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