  We report a new particle in cell (PIC) method based on the semi-implicit
approach. The novelty of the new method is that unlike any of its semi-implicit
predecessors at the same time retains the explicit computational cycle and
conserves energy exactly. Recent research has presented fully implicit methods
where energy conservation is obtained as part of a non linear iteration
procedure. The new method (referred to as Energy Conserving Semi-Implicit
Method, ECSIM), instead, does not require any non-linear iteration and its
computational cycle is similar to that of explicit PIC.
  The properties of the new method are: i) it conserves energy exactly to
round-off for any time step or grid spacing; ii) it is unconditionally stable
in time, freeing the user from the need to resolve the electron plasma
frequency and allowing the user to select any desired time step; iii) it
eliminates the constraint of the finite grid instability, allowing the user to
select any desired resolution without being forced to resolve the Debye length;
iv) the particle mover has a computational complexity identical to that of the
explicit PIC, only the field solver has an increased computational cost.
  The new ECSIM is tested in a number of benchmarks where accuracy and
computational performance are tested.
