  Neutral quantum absorbers in optical lattices have emerged as a leading
platform for achieving clocks with exquisite spectroscopic resolution. However,
the studies of these clocks and their systematic shifts have so far been
limited to atoms. Here, we extend this architecture to an ensemble of diatomic
molecules and experimentally realize an accurate lattice clock based on pure
molecular vibration. We evaluate the leading systematics, including the
characterization of nonlinear trap-induced light shifts, achieving a total
systematic uncertainty of $4.6\times10^{-14}$. The absolute frequency of the
vibrational splitting is measured to be 31 825 183 207 592.8(5.1) Hz, enabling
the dissociation energy of our molecule to be determined with record accuracy.
Our results represent an important milestone in molecular spectroscopy and
THz-frequency standards, and may be generalized to other neutral molecular
species with applications for fundamental physics, including tests of molecular
quantum electrodynamics and the search for new interactions.
