  Understanding the structure and dynamics of liquids is pivotal for the study
of larger spatiotemporal processes, particularly for glass-forming materials at
low temperatures. The so-called thermodynamic scaling relation, validated for
many molecular systems through experiments, offers an efficient means to
explore a vast range of time scales along a one-dimensional phase diagram.
Isomorph theory provides a theoretical framework for thermodynamic scaling
based on strong virial-potential energy correlations, but this approach is most
successful for simple point particles. In particular, isomorph theory has
resisted extension to complex molecular liquids due to the existence of
high-frequency intramolecular interactions. To elucidate the microscopic origin
of density scaling for molecular systems, we employ two distinct approaches for
coarse-graining in space or in time. The former eliminates fast degrees of
freedom by reducing a molecule to a center-of-mass-level description, while the
latter involves temporally averaged fluctuations or correlation functions over
the characteristic time scale. We show that both approaches yield a consistent
density scaling coefficient for ortho-terphenyl, which is moreover in agreement
with the experimental value. Building upon these findings, we derive the
density scaling relationship exhibiting a single-parameter phase diagram from
fully atomistic simulations. Our results unravel the microscopic nature
underlying thermodynamic scaling and shed light on the role of coarse-graining
for assessing the slow fluctuations in molecular systems, ultimately enabling
the extension of systematic bottom-up approaches to larger and more complex
molecular liquids that are experimentally challenging to probe.
