HubbardDensity

HubbardDensity computes the density matrix <math>\langle\Psi_{left}|c^\dagger_{i,\sigma} c_{j,\sigma'}|\Psi_{right}\rangle</math>. Its usage is identical to the one of HubbardSuperconductorOrderParameter. If more than one state are provided, HubbardDensity will then compute <math>\hat{P}_{N+2}c^\dagger_{i,\sigma} c_{j,\sigma'} \hat{P}_{N} c^\dagger_{j,\sigma'} c_{i,\sigma}</math> as defined in HubbardSuperconductorOrderParameter.