The way we perceive chemistry and chemical experiments has changed dramatically with the use of computers and simulations. Over the last two decades, computational chemistry has moved from an elitist activity to a standard investigative tool — and arguably to a prerequisite for publication, just as much as any spectroscopic characterization.

But computational chemists do not calculate the real world. We explain our colleagues' results from a simplified model that we ourselves created, and the quality of the answer is the quality of that model. I will illustrate this with recent examples from our own work — homogeneous catalysis and non-covalent interactions — where I discuss both the recent advances and the challenges that remain.