International audienceIn this paper, we revisit the monopole components of effective interactions forthe shell model. Without going through specific nuclei or shell gaps, universalroles of central, tensor, and spin–orbit forces can be proved, reflecting theintrinsic features of shell model effective interactions. For monopole matrixelements, even and odd channels of central force often have a canceling effect.However, for the contributions to the shell evolution, its even and oddchannels could have both positive or negative contributions, enhancing therole of central force on the shell structure. Tensor force is generally weakerthan central force. However, for the effect on shell evolutions, tensor force candominate or play a competitive role. A different systematics has been discoveredbetween T = 1 and 0 channels. For example, tensor force, wellestablished in the T = 0 channel, becomes uncertain in the T = 1 channel. Wecalculate the properties of neutron-rich oxygen and calcium isotopes in orderto study T = 1 channel interactions further. It is learned that the mainimprovements of empirical interactions are traced to the central force. For noncentralforces, antisymmetric spin–orbit (ALS) force, originated from manybodyperturbations or three-body force, could also play an explicit role. T = 1tensor forces are less constrained so their effect can differ in differentempirical interactions. The influence of tensor force may sometimes be canceledby many-body effects. For T = 0 channels of effective interactions,which is the main source of neutron–proton correlations, central and tensorforces are the leading components. For T = 1 channels, which can act betweenlike-particles, the request for many-body correlations could be moredemanding, so that the monopole anomaly of the T = 1 channel might be moreserious
