Nuclear Physics A523 (1991) 325-353 North-Holland zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA NEW EFFECTIVE INTERACTIONS FOR THE Oflp SHELL W.A. RICHTER and M.G. VAN DER MERWE Physics zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Depariment, University of SteNenbosch, Stellenbosch 7600, Republic zyxwvutsrqponmlkjihgfedcb of South Africa R.E. JULIES Physics Department, University of the W estern Cape, Private Bag Xl 7, Bellville, C. P. 7530, Republic of South Africa B.A. BROWN National Superconducting Cyclotron Laboratory and Department of Physics and Astronomy, Michigan State University, East Lansing, Mf 48824, USA Received 11 June 1990 (Revised 1 August 1990) Abstract: New two-body interactions are derived for nuclei in the lower part of the Oflp shell by fitting semi-empirical potential forms and two-body matrix elements to 61 binding and excitation energy data in the mass range 41 to 49. The shell-model calculations assumed a ?a core and valence nucleons distributed over the fuil fp space. Care was taken to exclude intruder states from the selected data set. The r.m.s. deviations between fitted and experimental energies of 176 keV have been achieved by varying only 6 two-body parameters and 4 single-particle energies in the method utilizing a modified surface one-boson exchange potential (MSOBEP) form. in an alternative fit an r.m.s. deviation of 163 keV has been obtained by fitting 12 linear combinations of single-particle energies and two-body matrix elements, while constraining the remaining two-body matrix elements to values of a G-matrix interaction. The results are comparable in quality to recent similar fits in the 1sOd shell. The effects of a mass dependence of the two-body matrix elements are also investigated. An excellent reproduction of ground-state magnetic moments and quadrupole moments is obtained with the new interactions. 1. Introduction The best currently available two-body effective interactions in nuclei are obtained using an unconstrained hamiltonian, in which all two-body matrix elements and single-particle energies are allowed to vary as free parameters in a fit to experimental data. This method will hereafter be referred to as the TBME method or the model-independent (MI) method. In the most comprehensive work employing the TBME method Wildenthal IS’) has been able to reproduce a selection of 447 IsOd shell binding and excitation energies with an r.m.s. deviation between experiment and theory of about 185 keV. The introduction of a mass dependence for the empirical IsOd interaction of Wildenthal was an impo~ant ingredient in its improved reproduc- tion of IsOd-shell observables over previous work 3). However, with such param- eterizations very little is generally revealed about the nature or form of the nucleon- nucleon interaction. It is also much more difficult to extract an unconstrained 0375Q474/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)