relentless is a Python package for executing molecular simulations in larger computational workflows. The simulation protocol is specified as a reproducible, human-readable recipe that is run natively in popular engines such as LAMMPS and HOOMD-blue. relentless also has robust features for optimization with simulations, including independent and dependent variables with automatic differentiation, objective functions of simulations, and optimization methods. All features are readily extensible through a clean hierarchy of abstract objects, enabling you to quickly use your own simulation code or optimization objective.