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effects went almost parallel with the further development of the shell model. As a result, a synthesis of the two aspects of nuclear dynamics, single particle, and collective behaviour, has become one of today's most interesting problems of nuclear structure. Collective aspects had for a long time dominated the thinking in nuclear physics, ever since in 1937 Bohr and Kalckar presented the liquid drop

We consider a linear antiferromagnetic chain of elastically coupled spins in the Hartree‐Fock approximation to the Heisenberg Hamiltonian. The nuclear dynamics is neglected. The instability of the system is investigated and the results are compared with the X‐Y model treated in I. It is shown that the Hartree‐Fock approximation is essentially equivalent to a renormalization of the exchange integral in the X‐Y model.