Very generally, the representation of a system is defined by all the variables that need to be determined based on input information, including the assignment of the system components to geometric objects (e.g. points, spheres, ellipsoids, and 3D Gaussian density functions). It is common to use a multiscale representation, where the system is represented using several resolution scales simultaneously. The spatial restraints will be applied to individual resolution scales as appropriate (for example, a cross-link may be applied at residue resolution, while an excluded volume restraint may act on a more coarse representation of the system, such as a spherical bead representing multiple residues).

In PMI, the resolution of a representation is simply the number of residues per spherical bead. For example, resolution 10 represents the system using a bead for every 10 residues, while resolution 1 uses a bead for each residue. An all-atom representation is assigned a resolution of 0.

Many PMI functions take a resolution parameter to specify which resolution to act on. These functions use IMP::pmi::tools::HierarchyDatabase to get the closest resolution if the exact resolution requested is not available.

Note: currently resolution is stored in the particle name. Ultimately it is intended to use the IMP::atom::Representation decorator for this purpose instead.