Effective quadrupole-quadrupole interaction from density functional theory

The density functional theory of nuclear structure provides a many-particle wave function that is useful for static properties, but an extension of the theory is necessary to describe correlation effects or other dynamic properties. We propose a procedure to extend the theory by mapping the properties of a self-consistent mean-field theory onto an effective shell-model Hamiltonian with quadrupole-quadrupole interaction. In this initial study, we consider the sd-shell nuclei Ne-20, Mg-24, Si-28, and Ar-36. The method is first tested with the USD shell-model Hamiltonian, using its mean-field approximation to construct an effective Hamiltonian and partially recover correlation effects. We find that more than half of the correlation energy is due to the quadrupole interaction. We then follow a similar procedure but using the SLy4 Skyrme energy functional as our starting point and truncating the space to the spherical sd shell. The constructed shell-model Hamiltonian is found to satisfy minimal consistency requirements to reproduce the properties of the mean-field solution. The quadrupolar correlation energies computed with the mapped Hamiltonian are reasonable compared with those computed by other methods. The method also provides a well-defined renormalization of the quadrupole operator in the shell-model space, the "effective charge" of the phenomenological shell model.