This is part of the colvar module

Calculate the total potential energy of the simulation box.

The potential energy can be biased e.g. with umbrella sampling [13] or with well-tempered metadynamics [19].

Notice that this CV could be unavailable with some MD code. When it is available, and when also replica exchange is available, metadynamics applied to ENERGY can be used to decrease the number of required replicas.

This ENERGY does not include long tail corrections. Thus when using e.g. LAMMPS "pair_modify tail yes" or GROMACS "DispCorr Ener" (or "DispCorr EnerPres"), the potential energy from ENERGY will be slightly different form the one of the MD code. You should still be able to use ENERGY and then reweight your simulation with the correct MD energy value.
Acceptance for replica exchange when ENERGY is biased is computed correctly only if all the replicas have the same potential energy function. This is for instance not true when using GROMACS with lambda replica exchange or with plumed-hrex branch.

The following input instructs plumed to print the energy of the system

Click on the labels of the actions for more information on what each action computes
tested on v2.8
ene: ENERGY 
the input for this action is the scalar output from one or more other actions.
Glossary of keywords and components