  Spin-crossover molecules having a low-spin ground state and a low-lying
excited high-spin state are promising components for molecular electronics. We
theoretically examine one-dimensional spin-crossover chain molecules of the
type of Fe-II triazole complexes. The existence of the additional
low-spin/high-spin degree of freedom leads to rich behavior already in the
ground state. We obtain the complete ground-state phase diagram, taking into
account an elastic nearest-neighbor interaction, a ferromagnetic or
antiferromagnetic exchange interaction between the magnetic ions, and an
external magnetic field. Ground-state energies are calculated with high
numerical precision using the density-matrix renormalization group. Besides
pure low-spin, highspin, and alternating low-spin/high-spin phases we obtain a
number of periodic ground states with longer periods, which we discuss in
detail. For example, for antiferromagnetic coupling there exists a dimer phase
with a magnetic unit cell containing two high-spin ions forming a spin singlet
and a single low-spin ion, which is stabilized by the energy gain for singlet
formation.
