  We propose a new framework by means of the dineutron condensate (DC) wave
function to describe the dineutron correlation, which is characterized by the
spatially strong correlation of a spin-zero neutron-neutron pair, in
neutron-rich nuclei with an active deformed core surrounded by valence
neutrons. Using the DC wave function for a 2$\alpha$+2n system, which
corresponds to a toy model for the $^{10}$Be system, we investigate the
neutron-neutron correlation around the $2\alpha$ core and discuss the mechanism
of the dineutron formation at the surface of finite nuclei. To investigate
dineutron correlations in realistic nuclear systems, we superpose the
antisymmetrized molecular dynamics (AMD) wave functions and the DC wave
functions. Applying the AMD+DC method to $^{10}$Be, we show effects of the DC
wave functions in the ground and excited $0^+$ states of $^{10}$Be and discuss
the dineutron correlation in them.
