  A method to separate a Slater determinant wave function with a two-center
neck structure into spatially localized subsystems is proposed, and its
potential applications are presented. An orthonormal set of spatially localized
single-particle wave functions is obtained by diagonalizing the coordinate
operator for the major axis of a necked system. Using the localized
single-particle wave functions, the wave function of each subsystem is defined.
Therefore, defined subsystem wave functions are used to obtain density
distributions, mass centers, and energies of subsystems. The present method is
applied to separations of Margenau--Brink cluster wave functions of $\alpha +
\alpha$, $^{16}$O + $^{16}$O, and $\alpha + ^{16}$O into their subsystems, and
also to separations of antisymmetrized molecular dynamics wave functions of
$^{10}$Be into $\alpha$ + $^6$He subsystems. The method is simple and
applicable to the separation of general Slater determinant wave functions that
have neck structures into subsystem wave functions.
