  ESPResSo 4.0 is an extensible simulation package for research on soft matter.
This versatile molecular dynamics program was originally developed for
coarse-grained simulations of charged systems Limbach et al., Comput. Phys.
Commun. 174, 704 (2006). The scope of the software has since broadened
considerably: ESPResSo can now be used to simulate systems with length scales
spanning from the molecular to the colloidal. Examples include, self-propelled
particles in active matter, membranes in biological systems, and the
aggregation of soot particles in process engineering. ESPResSo also includes
solvers for hydrodynamic and electrokinetic problems, both on the continuum and
on the explicit particle level. Since our last description of version 3.1
Arnold et al., Meshfree Methods for Partial Differential Equations VI, Lect.
Notes Comput. Sci. Eng. 89, 1 (2013), the software has undergone considerable
restructuring. The biggest change is the replacement of the Tcl scripting
interface with a much more powerful Python interface. In addition, many new
simulation methods have been implemented. In this article, we highlight the
changes and improvements made to the interface and code, as well as the new
simulation techniques that enable a user of ESPResSo 4.0 to simulate physics
that is at the forefront of soft matter research.
