  In order to examine the origin of octupole ordering in NpO2, we propose a
microscopic model constituted of neptunium 5f and oxygen 2p orbitals. To study
multipole ordering, we derive effective multipole interactions from the f-p
model by using the fourth-order perturbation theory in terms of p-f hopping
integrals. Analyzing the effective model numerically, we find a tendency toward
\Gamma_{5u} antiferro-octupole ordering.
