Description:VARIATIONAL APPROACH TO COLLECTIVE EXCITATIONS
Abstract:The study of correlations in quantum many-body systems is a subject of great interest among the physicists of various fields. Such an interest is testified by the continuous development of new methods which aim at improving the treatment of these correlations and so at making the microscopic description of collective excitation as reliable as possible. Among the approaches which have attracted more attention in this field, a preeminent role is certainly played by the random phase Approximation (RPA).In this theory, the excitated states of a many-body systems are described in terms of phonon creation operators acting on the ground state. This state is in turn defined as the vacuum of the adjoints of these phonon operators. Due to this definition, an explicit expression of the RPA ground state can be found only by approximating the phonon operators to bosons.The approach we have developed maintains an RPA-like structure since excited states still result from the action of creation operators on the ground state. In our approach, however, this state is explicitly derived by means of an iterative variational procedure and without resorting to any bosonic approximation. Of course, making use of a (complex) wave function for the ground state makes calculations considerably more involved than in the RPA case. In such a context, then, the use of the PI2S2 grid infrastructure has revealed to be crucial in order to considerably reduce the time needed by the calculations by therefore allowing a more efficient examination of the preliminar results which look quite promising.