  Intermolecular van der Waals interactions are central to chemical and
physical phenomena ranging from biomolecule binding to soft-matter phase
transitions. However, there are currently very limited approaches to manipulate
van der Waals interactions. In this work, we demonstrate that strong
light-matter coupling can be used to tune van der Waals interactions, and,
thus, control the thermodynamic properties of many-molecule systems. Our
analyses reveal orientation dependent single molecule energies and interaction
energies for van der Waals molecules (for example, H$_{2}$). For example, we
find intermolecular interactions that depend on the distance between the
molecules $R$ as $R^{-3}$ and $R^{0}$. Moreover, we employ non-perturbative
\textit{ab initio} cavity quantum electrodynamics calculations to develop
machine learning-based interaction potentials for molecules inside optical
cavities. By simulating systems ranging from $12$ H$_2$ to $144$ H$_2$
molecules, we demonstrate that strong light-matter coupling can tune the
structural and thermodynamic properties of molecular fluids. In particular, we
observe varying degrees of orientational order as a consequence of
cavity-modified interactions, and we explain how quantum nuclear effects,
light-matter coupling strengths, number of cavity modes, molecular
anisotropies, and system size all impact the extent of orientational order.
These simulations and analyses demonstrate both local and collective effects
induced by strong light-matter coupling and open new paths for controlling the
properties of molecular clusters.
