I got it. Thank you for your reply.

Please distinguish between spin density and spin population, they are very different. What you want to obtain is spin population of fragments.

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: 1.52405406

The spin population of the fragment, namely the sum of spin population of O8 and H9, is the 1.524 shown above.

Hi,
I am interested to find out the spin density on each fragment of the molecule. Is this possible to calculate the value of spin density from each fragment?