Nucleon-pair approximation with particle-hole excitations

In this paper, the formalism for the nucleon-pair approximation (NPA) with particle-hole excitations is developed. By using this approach, one is able to perform multiple-major-shell calculations. In this formulation, we use a particle-hole mixed representation, i.e., the particle-hole conjugate transformation is used for operators of the lower major shells, while operators of the upper major shells remain unchanged. We consider collective valence-particle pairs in the upper major shells, collective valence-hole pairs in the lower major shells, and collective pairs consisting of one valence particle in the upper shells and one valence hole in the lower shells to construct our model space. Matrix elements for the shell-model Hamiltonian, both for effective interactions and for phenomenological pairing plus multipole-multipole interactions, are derived in the nucleon-pair basis. As a special case, analytical formulas for doubly magic nuclei with excitations up to two-particle–two-hole are presented. To exemplify this approach, we calculate ${}^{100}\mathrm{Sn}$ considering both proton and neutron up to four-particle–four-hole excitations, where valence particles are in the 50-82 major shell and valence holes in the 28-50 major shell, with the low-momentum nucleon-nucleon interaction derived from the CD-Bonn potential.

Figure 1

Distributions of nonzero matrix elements for (a) $U_i$ with $i=1,2,3,4$ and for (b) $V_j$ with $j=1,...,16$. The bra vectors of the matrix elements in the first, second, third, fourth, etc. rows correspond to 0p0h, 1p1h, 2p2h, 3p3h, etc. excitations, respectively. The ket vectors of the matrix elements in the first, second, third, fourth, etc. columns correspond to 0p0h, 1p1h, 2p2h, 3p3h, etc. excitations, respectively.