A study of the p-d model by means of the *COM* is
reported in
Refs. [1,90,91,92,93].
In particular the following properties have been computed:

- The thermodynamic properties (i.e., chemical potential, double
occupancy, local magnetic moment, hybridization)
[90,93]
- The single-particle properties (i.e., energy spectra, density of states, Fermi surface, charge transfer susceptibility) [1,92,93]

The *COM* results have been compared with the ones
obtained by:

- Numerical techniques (i.e., Exact diagonalization, Lanczos and quantum
Monte Carlo) [94,95,96]
- Exact results on cluster [1]

The p-d model has been studied in the context of the
*COM* by considering different approximations. The
so-called reduced p-d model, where the transitions to the lower
Hubbard band are neglected, has been analyzed in
Refs. [1,90,91]. In
these papers different choices of the basic composite fields have
been taken. In Ref. [1] the effect of
the local antiferromagnetic correlations on the electronic
properties, density of states and band dispersions, has been
analyzed as a function of the hole doping. The chosen basic
operators reproduce the exact single-site results.

In Ref. [90] the spin correlator has been self-consistently calculated by means of a sum rule with the content of the Pauli principle; the calculated local magnetic moment agrees well with the data obtained by numerical simulation.

In Ref. [91] we have enlarged the basic field by considering on the same foot both spin and charge correlations. As a result the set of equations to determine all the correlators closes and a fully self-consistent formulation is obtained.

The complete p-d model has been studied in Refs. [92,93] in a four-pole approximation. The density of states shows the Zhang-Rice singlet band situated around the chemical potential. The dispersion of this band gives a Fermi surface which is in good agreement with the experimental results for . Several quantities, the local magnetic moment, the chemical potential, the d- and p- particle densities and the charge-transfer susceptibility, are in excellent agreement with the numerical results.