  Enhanced sampling algorithms have emerged as powerful methods to extend the
utility of molecular dynamics simulations and allow the sampling of larger
portions of the configuration space of complex systems in a given amount of
simulation time. This review aims to present the unifying principles and
differences of many of the computational methods currenly used for enhanced
sampling in molecular simulations of biomolecules, soft matter and molecular
crystals. Indeed, despite the apparent abundance and divergence of such
methods, the principles at their core can be boiled down to a relatively
limited number of statistical and physical principles. To enable comparisons,
the various methods are introduced using similar terminology and notation. We
then illustrate in which ways many different methods combine principles from a
smaller class of enhanced sampling concepts. This review is intended for
scientists with an understanding of the basics of molecular dynamics
simulations and statistical physics who want a deeper understanding of the
ideas that underlie various enhanced sampling methods and the relationships
between them. This living review is intended to be updated to continue to
reflect the wealth of sampling methods as they continue to emerge in the
literature.
