We formulate the computational method to calculate the overlap
integral of the shell-model and cluster-model
configurations. The framework is applied to the systems of the core
plus two neutrons (n), and the magnitude of the overlap of the
shell model configuration (core + n + n) and the
di-neutron cluster one (core + 2n) are explored.
The overlap integrals are systematically calculated in the
systems with the jj-closed cores, and we have found
that the enhancement due to the occupation of the s or p
orbit occurs in the systematic calculations.