T=1 effective interaction in the sd shell

The effective interaction in T=1 two-body states in the sd shell is parameterized in terms of radial matrix elements of the effective potential in states of relative orbital angular momenta of two nucleons. Seven of these parameters are needed to define all the T=1 matrix elements with the assumption of a central effective potential. The values of these parameters are determined by making a least-squares fit to 16 observed level energies in the oxygen isotopes and are then compared with values calculated from various realistic free nucleon-nucleon interactions. It is shown that a major effect of core excitation and consequent renormalization of the two-body matrix elements, so forcefully pointed out by Kuo and Brown, consists of a strong modification of the contributions of the triplet-odd force. Two-body matrix elements calculated with our parameters are also found to agree well with those determined by Glaudemans et al. from a least-squares fit to observed level schemes of nuclei with A=29-40