Deformation in the Second Half of thes−dShell

A simple approximate procedure for perturbing the self-consistent Hartree-Fock equations in an enlarged single-particle space is given. Using phenomenological interactions and observed single-particle splittings, the second half of the $s-d$ shell is shown to require pear-shaped intrinsic states, involving excitations from the $s-d$ to the $p-f$ shell.