Effects of two-site composite excitations in the Hubbard model

TL;DR

This paper investigates the Hubbard model's electronic states using an enhanced composite operator approach that includes two-site excitations, leading to improved accuracy over traditional methods.

Contribution

It introduces a new basis incorporating two-site composite excitations and a symmetry-preserving reduction, improving the modeling of the Hubbard model.

Findings

Better agreement with numerical simulations for double occupancy.

Improved over the two-pole approximation.

Maintains particle-hole symmetry.

Abstract

The electronic states of the Hubbard model are investigated by use of the Composite Operator Method. In addition to the Hubbard operators, two other operators related with two-site composite excitations are included in the basis. Within the present formulation, higher-order composite excitations are reduced to the chosen operatorial basis by means of a procedure preserving the particle-hole symmetry. The positive comparison with numerical simulations for the double occupancy indicates that such approximation improves over the two-pole approximation.