Atomistics is a branch of science that has grown out of the application of quantum mechanics to molecular and condensed matter systems. For much of the past five or so decades, these two sciences have progressed along parallel tracks, yet recently these tracks have started to intersect and intertwine. Traditionally molecular codes such as Gaussian have added periodic boundary conditions and are now being used to compute the band structures of insulating solids, and plane-wave codes traditionally used only for solid-state systems are now being applied to look at nanoparticles and molecular systems. This convergence is more or less the inevitable consequence of the use of faster computational systems that can stretch the limits of algorithms beyond their original domains. An additional factor that has hastened this convergence is the increasing awareness of the relevance of interfacial systems and defects, which tend to fit into neither the ideal periodic boundary conditions approach, nor the isolated molecular cluster reduction.
Atomistics is, therefore, the application of quantum mechanics based models for systems beyond mere molecules or bulk materials, but encompasses defects, intermolecular complexes, and surfaces. Atomistics lends itself to not only static calculations of molecular properties (which can alone be highly insightful) but also to dynamic or kinetic simulations. It does not only utilize wavefunction/electronic-structure based approaches, but can also leverage interatomic (or pair-wise) potentials. Such potentials are often refined by the use of high quality quantum mechanical datasets. Atomistics provides an atomically resolved computational microscope into the world of materials and molecular processes. Like any scientific method, its strengths and limitations need to be carefully understood, and, like any scientific method, experiments should be intentionally structured so as to allow for hypothesis refinement.