# Adaptive Cross-Approximation (ACA)

The adaptive cross-approximation (ACA) is a fast method similar to the multilevel fast multipole method (MLFMM) but is also applicable to low-frequency problems or when using a special Green’s function.

The adaptive cross-approximation (ACA) approximates the impedance matrix by constructing a sparse H-matrix (only a few selected elements are computed). The ACA is similar to the MLFMM in the sense that they are both used for large models where the method of moments has become too resource intensive, but they are quite different in implementation and applications. Models with many unknowns, where the model is electrically small (less than a wavelength) can be solved with ACA. For electrically large structures (multiple wavelengths) the multilevel fast multipole method is much better suited.