  Molecular simulations are playing an ever increasing role, finding
applications in fields as varied as physics, chemistry, biology and material
science. However, many phenomena of interest take place on time scales that are
out of reach of standard molecular simulations. This is known as the sampling
problem and over the years several enhanced sampling methods have been
developed to mitigate this issue. We propose a unified approach that puts on
the same footing the two most popular families of enhanced sampling methods,
and paves the way for novel combined approaches. The on-the-fly probability
enhanced sampling method provides an efficient implementation of such
generalized approach, while also focusing on simplicity and robustness.
