A multi-reference coupled cluster formalism for the study of the lowest eigenstates of Hubbard Hamiltonians

Taking advantage of the simplicity of the Hubbard or Pariser Parr Pople Hamiltonian, it is possible to define a multi-reference few-state coupled cluster expansion. The reference functions are all the most neutral valence-bond (VB) determinants and the targets are the lowest eigenstates. Test calculations show that the method gives reliable results even when the |t/U| ratio is as large as 1, and provides excellent additional roots.