Granularity

Granularity (also called "graininess", the quality of being grainy) is the extent to which a material or system is composed of distinguishable pieces or grains. It can either refer to the extent to which a larger entity is subdivided, or the extent to which groups of smaller indistinguishable entities have joined together to become larger distinguishable entities. For example, a kilometer broken into centimeters has finer granularity than a kilometer broken into meters. In contrast, molecules of photographic emulsion may clump together to form distinct noticeable granules, reflecting coarser granularity.

Coarse-grained materials or systems have fewer, larger discrete components than fine-grained materials or systems. A coarse-grained description of a system regards large subcomponents while a fine-grained description regards smaller components of which the larger ones are composed.

The concepts granularity, coarseness, and fineness are relative, used when comparing systems or descriptions of systems. An example of increasingly fine granularity: a list of nations in the United Nations, a list of all states/provinces in those nations, a list of all cities in those states, etc.

The terms fine and coarse are used consistently across fields, but the term granularity itself is not. For example, in investing, more granularity refers to more positions of smaller size, while photographic film that is more granular has fewer and larger chemical "grains." Similarly, sugar that is more granular has fewer and larger grains.

A fine-grained description of a system is a detailed, exhaustive, low-level model of it. A coarse-grained description is a model where some of this fine detail has been smoothed over or averaged out. The replacement of a fine-grained description with a lower-resolution coarse-grained model is called coarse graining. (See for example the second law of thermodynamics)

In molecular dynamics, coarse graining consists of replacing an atomistic description of a biological molecule with a lower-resolution coarse-grained model that averages or smooths away fine details. Coarse-grained models have been developed for investigating the longer time- and length-scale dynamics that are critical to many biological processes, such as lipid membranes and proteins. These concepts not only apply to biological molecules but also inorganic molecules. Coarse graining may remove certain degrees of freedom (e.g. vibrational modes between two atoms) or represent the two atoms as a single particle. The ends to which systems may be coarse grained is simply bound by the accuracy in the dynamics and structural properties one wishes to replicate. This modern area of research is in its infancy, and although it is commonly used in biological modeling, the analytic theory behind it is poorly understood.

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