Non-degenerate shell-model effective interactions from the Okamoto-Suzuki and Krenciglowa-Kuo iteration methods

Abstract: We present calculations of shell-model effective interactions for both degenerate and non-degenerate model spaces using the Krenciglowa-Kuo (KK) and the extended Krenciglowa-Kuo iteration method recently developed by Okamoto, Suzuki {\it et al.} (EKKO). The starting point is the low-momentum nucleon-nucleon interaction $V_{low-k}$ obtained from the N$^3$LO chiral two-nucleon interaction. The model spaces spanned by the $sd$ and $sdpf$ shells are both considered. With a solvable model, we show that both the KK and EKKO methods are convenient for deriving the effective interactions for non-degenerate model spaces. The EKKO method is especially desirable in this situation since the vertex function $\hat Z$-box employed therein is well behaved while the corresponding vertex function $\hat Q$-box employed in the Lee-Suzuki (LS) and KK methods may have singularities. The converged shell-model effective interactions given by the EKKO and KK methods are equivalent, although the former method is considerably more efficient. The degenerate $sd$-shell effective interactions given by the LS method are practically identical to those from the EKKO and KK methods. Results of the $sd$ one-shell and $sdpf$ two-shell calculations for $^{18}$O, $^{18}$F, $^{19}$O and $^{19}$F using the EKKO effective interactions are compared, and the importance of the shell-model three-nucleon forces is discussed.