  First-order transitions of system where both lattice site occupancy and
lattice spacing fluctuate, such as cluster crystals, cannot be efficiently
studied by traditional simulation methods. These methods necessarily fix one of
these two degrees of freedom, but this difficulty is surmounted by the
generalized [N]pT ensemble [J. Chem. Phys. 136, 214106 (2012)]. Here it is
shown that histogram reweighting and the [N]pT ensemble can be used to study an
isostructural transition between cluster crystals of different occupancy in the
generalized exponential model of index 4 (GEM-4). Extending this scheme to
finite-size scaling studies also allows to accurately determine the critical
point parameters and to verify that it belongs to the Ising universality class.
