  We develop a Monte Carlo event generator for Higgs Boson pair production
merged to exact one-jet matrix elements. The matrix elements are generated with
OpenLoops and event generation is performed with the HERWIG++ general-purpose
event generator. This allows us to simulate fully-exclusive hadronic final
states with accurate description of the kinematics of the leading jet in
conjunction with a parton shower. We use the implementation to examine in
detail the systematic uncertainties which result from the merging procedure. We
assess the magnitude of the impact of the merging on experimental searches of
Standard Model di-Higgs production that aim to constrain the Higgs boson
self-coupling. We find that the use of a merged sample can reduce theoretical
systematic uncertainties in the efficiencies of cuts on certain observables.
This constitutes the most accurate simulation of the process available to date.
The Monte Carlo event generator developed for this project is available as an
add-on to the HERWIG++ event generator at http://www.itp.uzh.ch/~andreasp/hh
