  Solid-solid collapse transition in open framework structures is ubiquitous in
nature. The real difficulty in understanding detailed microscopic aspects of
such transitions in molecular systems arises from the interplay between
different energy and length scales involved in molecular systems, often
mediated through a solvent. In this work we employ Monte Carlo (MC) simulations
to study the collapse transition in a model molecular system interacting via
both isotropic as well as anisotropic interactions having different length and
energy scales. The model we use is known as Mercedes-Benz (MB) which for a
specific set of parameters sustains three solid phases: honeycomb, oblique and
triangular. In order to study the temperature induced collapse transition, we
start with a metastable honeycomb solid and induce transition by heating. High
density oblique solid so formed has two characteristic length scales
corresponding to isotropic and anisotropic parts of interaction potential.
Contrary to the common believe and classical nucleation theory, interestingly,
we find linear strip-like nucleating clusters having significantly different
order and average coordination number than the bulk stable phase. In the early
stage of growth, the cluster grows as linear strip followed by branched and
ring-like strips. The geometry of growing cluster is a consequence of the
delicate balance between two types of interactions which enables the dominance
of stabilizing energy over the destabilizing surface energy. The nuclei of
stable oblique phase are wetted by intermediate order particles which minimizes
the surface free energy. We observe different pathways for pressure and
temperature induced transitions.
