ALD approaches use molecular precursors to deposit thin films. Often, also the co-reactant is a molecule (mostly H2O) or a molecular gas (e.g., O2, O3). Even in energy-enhanced approaches (e.g., plasma), the reactive species are created from molecules. In inhibitor-based area-selective approaches, the inhibitor is also mostly a molecular species (Tab. 1). Thus, the modelling of molecular properties and reactivity is crucial for the understanding of AS-ALD processes.