At the dawn of the 20th Century, scientists were struggling to understand the nature of matter. Nobody knew whether molecules really existed, or why elements might form them. Gradually, pieces of evidence from many different fields, including early particle physics, electrochemistry, thermodynamics and astrophysics began to paint a picture of matter that was quite different from what classical physics experience taught us was correct at human scales. This was quantum physics, and its peak achievement was a description of the chemical bond. One of the basic principles of quantum physics is that energy can only be added or subtracted to or from a system in minimum fixed ‘discrete’ amounts, and in chemical bonds this causes the electrons to move between molecular orbitals, the allowed energy states around the nucleus. The orbitals that the electrons occupy affect how – or even if – the molecule holds together.The equations that calculate the energies needed to move electrons around boil down to wave mechanics and something called the Schrödinger equation. For simple molecules, solving the Schrödinger equation is difficult enough, and the only molecule that scientists could do this for before computers was H2 – the simplest molecule. The first computers – which according to modern definition were supercomputers – enabled the calculations needed to figure out what was happening in larger molecules. Because the time taken to perform the quantum physics calculation for more complex molecules increases as the cube of the number of atoms they contain, the advancement of supercomputer power was for many decades more than matched by science’s need to understand ever-more complex molecular systems.