Because pressure is a classic thermodynamic state variable, it has appeared in equations since the beginning of thermodynamics. Later, in the first half of the 20 th century, seminal contributions have appreciated the role of pressure in quantum mechanics and its influence on chemical reactivity. Turning to high-pressure ab initio simulations, early work presented Self-Consistent Field (SCF) calculations of quantum mechanical systems under pressure, e.g., a hydrogen atom in a box. Placing atoms and small molecules in boxes confined by wall potentials is a fruitful endeavor and such approaches are still being developed today (Section 2.2).