  We present in this work the implementation of the Energy Conserving
Semi-Implicit Method in a parallel code called ECsim. This new code is a
three-dimensional, fully electromagnetic particle in cell (PIC) code. It is
written in C/C++ and uses MPI to allow massive parallelization. ECsim is
unconditionally stable in time, eliminates the finite grid instability, has the
same cycle scheme as the explicit code with a computational cost comparable to
other semi-implicit PIC codes. All this features make it a very valuable tool
to address situations which have not been possible to analyze until now with
other PIC codes. In this work, we show the details of the algorithm
implementation and we study its performance in different systems. ECsim is
compared with another semi-implicit PIC code with different time and spectral
resolution, showing its sability to address situations where other codes fail.
