Has anyone written an ALPS operator for the operator S^2 for a general number of spins in a system?

For example, I would like to list the Sztotal and S^2 values for different energy levels calculated.

The S^2 operator can be written as

S^2 = (S_1 + S_2 + S_3 +…)^2

where S_1, S_2, etc. are the vector operators.

for two spins one can write this as

S^2 = (S_1 + S_2)^2

      =    (S_1)^2 +  (S_2)^2 + 2 S_1 \dot S_2

       = s(s+1) + s(s+1) + 2S_{1z}S_{2z} + S_{1+}S_{2-} + S_{1-}S_{2+}

But I don’t see how to generalize this for a system of many spins using the ALPS “spin” sitebasis

and basis in order to have this operator value listed in the output calculations.

Any help would be appreciated. 

Thanks,

Matthew B. Stone

Neutron Scattering Science Division

Oak Ridge National Laboratory

PO box 2008 MS6475

Oak Ridge, TN 37831-6475

Phone: 1-865-202-6898

Fax: 1-865-574-6080