There are many different ways to compute spin population. As an example, we calculate spin population of OH fragment of examples\ethanol_triplet.fch, boot up Multiwfn and input
examples\ethanol_triplet.fch
15 //Fuzzy atomic space analysis
-5 //Define the atoms to be considered in options 1, 2, 13
8,9 //The atoms in the fragment
1 //Perform integration in fuzzy atomic spaces for a real space function
5 //Spin density
Then spin density is integrated within spaces of atoms 8 and 9, you can see
Atomic space Value % of sum % of sum abs
1(C ) 0.00000000 0.000000 0.000000
2(H ) 0.00000000 0.000000 0.000000
3(H ) 0.00000000 0.000000 0.000000
4(H ) 0.00000000 0.000000 0.000000
5(C ) 0.00000000 0.000000 0.000000
6(H ) 0.00000000 0.000000 0.000000
7(H ) 0.00000000 0.000000 0.000000
8(O ) 1.06453241 69.848730 69.848730
9(H ) 0.45952165 30.151270 30.151270
Summing up above values: 1.52405406
Summing up absolute value of above values: 1.52405406The spin population of the fragment, namely the sum of spin population of O8 and H9, is the 1.524 shown above.
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