This program deals with two holes moving around in

1. a periodic lattice of CuO2 unit cells
2. an impurity model: Cu site (only one) and the O square lattice surrounding it.

The variational space is constructed and then the Hamiltonian matrix is generated and diagonalized with Lanczos.

Difference of impurity model from the periodic lattice model calculation:

1. There is no k-dependence
2. The impurity model keeps the lattice structure the same as CuO2 but now there is only one Cu-site at (0,0)