Complex magnetic domain structures and the energy barriers between them are responsible
for pseudo-single-domain phenomena in rock magnetism and contribute significantly to the magnetic
remanence of paleomagnetic samples. This article introduces MERRILL, an open source software package
for three-dimensional micromagnetics optimized and designed for the calculation of such complex
structures. MERRILL has a simple scripting user interface that requires little computational knowledge to use
but provides research strength algorithms to model complex, inhomogeneous domain structures in
magnetic materials. It uses a finite element/boundary element numerical method, optimally suited for
calculating magnetization structures of local energy minima (LEM) in irregular grain geometries that are of
interest to the rock and paleomagnetic community. MERRILL is able to simulate the magnetic characteristics
of LEM states in both single grains, and small assemblies of interacting grains, including saddle-point paths
between nearby LEMs. Here the numerical model is briefly described, and an overview of the scripting
language and available commands is provided. The open source nature of the code encourages future
development of the model by the scientific community.