The calculation of positive ancl negative spin densities in aromatic free radicals

A simple method of calculating the unpaired π electron spin density is proposed which equates the unpaired spin density to the difference between a density of n-1 electrons of a spin and a density of n electrons of β spin. The α spin density is found from an SCF-Pariser-Parr type calculation for 2(n+l) electrons in n + 1 closed shells and the β spin density from a calculation on 2n electrons in n, closed shells. Positive and negative spin densities are found which are in excellent agreement with experiment. These results are compared to spin densities calculated by other methods, and an improvement is noted in the representative cases considered.