  The long-range electrostatic interactions between molecules depend strongly
on their relative orientation, which manifests as a rotational state
dependence. Interactions between molecules in the same rotational quantum state
are well-known attractive rotational van der Waals interactions. Interactions
in rotational states that differ by one quantum show resonant dipole-dipole
interactions. We show that where molecules are in rotational states that differ
by more than one quantum, they exhibit repulsive van der Waals interactions. At
temperatures below a millikelvin, this effect can reduce collisional loss by
multiple orders of magnitude. These repulsive interactions lead to applications
in quantum simulation and impurity physics with ultracold polar molecules.

Atomic Physics;Quantum Gases;Chemical Physics;Quantum Physics