The drip line of F isotopes is discussed from the viewpoint of a unified description of exotic nuclei with $N\sim 20.$ The structure of those nuclei is described by means of large-scale shell model calculations, most of which are Monte Carlo shell model. It is shown that, due to the narrowing of the $N=20\mathrm{}$ shell gap towards smaller Z, the mixing among normal, intruder, and higher intruder configurations can be enhanced significantly, and can produce crucial effects, including the F drip line located extremely far from the O drip line. The feasibility of Monte Carlo shell model is demonstrated for systems with an odd number of fermions.

• Received 14 September 2000

DOI:https://doi.org/10.1103/PhysRevC.64.011301