Coupling models at different spatial or temporal resolutions while making their interfaces, units and conservation requirements explicit.
Equation-free modeling performs coarse-scale calculations using short runs of a finer-scale simulator when an explicit coarse evolution equation is unavailable. A lifting step constructs microscopic states consistent with coarse variables; simulation advances them; restriction measures the resulting coarse state. These operations define a coarse time-stepper. [ref]
Equation-Free, Coarse-Grained Multiscale Computation: Enabling Microscopic Simulators to Perform System-Level Analysis by Yannis G. Kevrekidis and others. 2003, lifting, restriction and coarse time-stepper framework
For example, microscopic cell states can be initialized to match a population density, evolved briefly, and averaged to estimate a coarse change rate. That estimate can feed a longer projective step or a steady-state calculation. The method needs suitable coarse variables and separation between fast relaxation and slow evolution. If unresolved memory remains important, two microscopic preparations with the same coarse value can evolve differently, invalidating the assumed closure.
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Multiscale modeling is an approach used in various scientific and engineering disciplines to study complex systems that exhibit behavior across different scales, such as spatial scales (ranging from atomic to macroscopic) or temporal scales (ranging from picoseconds to years). The objective of multiscale modeling is to effectively link and integrate information and phenomena occurring at these different scales to provide a more comprehensive understanding of the system.