  The energy spectra of light-mass kaonic nuclei were investigated using the
theoretical framework of the $0s$-orbital model with zero-range $\bar{K} N$ and
$\bar{K}\bar{K}$ interactions of effective single-channel real potentials. The
energies of the $\bar{K} NN$, $\bar{K} NNN$, $\bar{K} NNNN$, $\bar{K}\bar{K}
N$, and $\bar{K}\bar{K} NN$ systems were calculated in the cases of weak- and
deep-binding of the $\bar{K} N$ interaction, which was adjusted to fit the
$\Lambda(1405)$ mass with the energy of the $\bar{K} N$ bound state. The
results qualitatively reproduced the energy systematics of kaonic nuclei
calculated via other theoretical approaches. In the energy spectra of the
$\bar{K} NN$ and $\bar{K}\bar{K} NN$ systems, the lowest states $\bar{K}
NN(J^\pi,T=0^-,1/2)$ and $\bar{K}\bar{K} NN(0^+,0)$ were found to have binding
energies approximately twice and four times as large as that of the $\bar{K}
N(1/2^-,0)$ state, respectively. Higher $(J^\pi,T)$ states including $\bar{K}
NN(1^-,1/2)$, $\bar{K}\bar{K} NN(0^+,1)$, and $\bar{K}\bar{K} NN(1^+,1)$ were
predicted at energies of 9--25 MeV below the antikaon-decay threshold. The
effective $\Lambda(1405)$-$\Lambda(1405)$ interaction in the $\bar{K}\bar{K} N
N$ system was also investigated via a $\bar{K} N+\bar{K} N$-cluster model.
Strong and weak $\Lambda(1405)$-$\Lambda(1405)$ attractions were obtained in
the $S^\pi=0^+$ and $S^\pi=1^-$ channels, respectively. The
$\Lambda(1405)$-$\Lambda(1405)$ interaction in the $$\bar{K}\bar{K} NN$ system
was compared with the effective $d$-$d$ interaction in the $NNNN$ system, and
the properties of dimer-dimer interactions in hadron and nuclear systems were
discussed.
