peratom

Peratom is a normalization concept used in physics, chemistry, and materials science to express a quantity on a per-atom basis. It is defined by dividing the total value of the quantity by N, the number of atoms in the considered system, so that per-atom value = total value / N. This approach facilitates meaningful comparisons across samples that differ in size or composition and is commonly applied to energies, charges, densities, and related properties.

Typical applications include per-atom energy in simulations, per-atom charge distributions, or per-atom catalytic activity. In computational work, N is taken as the number of atoms explicitly represented in the model or as the number of atoms per unit cell, depending on the context. When using per-atom normalization, it is important to specify how N is determined and which reference frame or convention is being used, as different choices can influence the resulting values.

Per-atom quantities are useful for comparing materials and systems of varying sizes, but they can obscure cooperative effects or size-dependent phenomena that arise from collective interactions. They are derived quantities and not direct observables, so their interpretation rests on the chosen normalization scheme and the context of the study. Related concepts include molar quantities, specific properties, and other normalization methods used in simulations and data analysis.