MG5_aMC+Herwig++ Always Herwig++ default PDF is used?

Dear MG5_aMC experts,

I think there is a missing parameter in your automatic script if LHE files are generated by MG5_aMC@NLO and then showered with Herwig++.

You are defining the PDF according to the hard subprocess also in the Herwig++ IN.file, however, it seems to me that you never define that this PDF should be used by the LHEReader or in Herwig++ at all. So the PDF is defined but never used.

I maybe wrong, but in my opinion the following two lines are missing:

set /Herwig/EventHandlers/LHEReader:PDFA /LHAPDF/MCNLOPDF
set /Herwig/EventHandlers/LHEReader:PDFB /LHAPDF/MCNLOPDF

In this way the PDF from the hard subprocess will be also used in Herwig++ for reading in the LHE level events while still the default shower and UE PDF is used.

If you want to use the PDF for everything in Herwig++ which is not recommend since this could mean that you use a NLO PDF for the shower and UE and / or use a PDF which does not correspond to the tune, you should add the following two lines to your Herwig++ IN.file script:

set /Herwig/Particles/p+:PDF /LHAPDF/MCNLOPDF
set /Herwig/Particles/pbar-:PDF /LHAPDF/MCNLOPDF

Here is your automatic script which I obtained using MG5_aMC@NLO+Herwig++ showering:

#
# Create the Les Houches handler for MC@NLO
#

library LesHouches.so

# --------------------------------------------------------------------- #
# 1.) NECESSARY SETTINGS FOR RUNNING WITH MC@NLO EVENTS (DO NOT MODIFY)

set /Herwig/Shower/Evolver:HardVetoMode 1
set /Herwig/Shower/Evolver:HardVetoScaleSource 1
set /Herwig/Shower/Evolver:MECorrMode 0

# create the Handler & Reader
create ThePEG::LesHouchesFileReader /Herwig/EventHandlers/LHEReader
create ThePEG::LesHouchesEventHandler /Herwig/EventHandlers/LHEHandler
insert /Herwig/EventHandlers/LHEHandler:LesHouchesReaders 0 /Herwig/EventHandlers/LHEReader

set /Herwig/EventHandlers/LHEReader:AllowedToReOpen 0
set /Herwig/EventHandlers/LHEReader:MomentumTreatment RescaleEnergy
set /Herwig/EventHandlers/LHEReader:WeightWarnings 0

set /Herwig/EventHandlers/LHEHandler:WeightOption VarNegWeight
set /Herwig/EventHandlers/LHEHandler:PartonExtractor /Herwig/Partons/QCDExtractor
set /Herwig/EventHandlers/LHEHandler:CascadeHandler /Herwig/Shower/ShowerHandler
set /Herwig/EventHandlers/LHEHandler:HadronizationHandler /Herwig/Hadronization/ClusterHadHandler
set /Herwig/EventHandlers/LHEHandler:DecayHandler /Herwig/Decays/DecayHandler
insert /Herwig/EventHandlers/LHEHandler:PreCascadeHandlers 0 /Herwig/NewPhysics/DecayHandler

set /Herwig/Generators/LHCGenerator:EventHandler /Herwig/EventHandlers/LHEHandler
set /Herwig/Generators/LHCGenerator:NumberOfEvents 200000
set /Herwig/Generators/LHCGenerator:RandomNumberGenerator:Seed 0
set /Herwig/Generators/LHCGenerator:PrintEvent 2
set /Herwig/Generators/LHCGenerator:MaxErrors 20001
set /Herwig/Generators/LHCGenerator:DebugLevel 1

# Define PDF from MCatNLO.inputs PDFSET number
mkdir /LHAPDF
create ThePEG::LHAPDF /LHAPDF/MCNLOPDF ThePEGLHAPDF.so
set /LHAPDF/MCNLOPDF:PDFName NNPDF30_nlo_as_0118

# DEFINE THE INPUT EVENT FILE
set /Herwig/EventHandlers/LHEReader:FileName events.lhe

# --------------------------------------------------------------------- #
# 2.) DEFINE PHYSICS PARAMETERS FROM FILE <MCatNLO_MadFKS.inputs> (DO NOT MODIFY)

# TODO set the correct incoming particles

# set masses and widths
set /Herwig/Particles/e-:NominalMass 0.000510999
set /Herwig/Particles/e+:NominalMass 0.000510999
set /Herwig/Particles/mu-:NominalMass 0.105658
set /Herwig/Particles/mu+:NominalMass 0.105658
set /Herwig/Particles/tau-:NominalMass 1.77699
set /Herwig/Particles/tau+:NominalMass 1.77699
set /Herwig/Particles/W+:NominalMass 80.419002
set /Herwig/Particles/W-:NominalMass 80.419002
set /Herwig/Particles/Z0:NominalMass 91.199997
set /Herwig/Particles/t:NominalMass 173.5
set /Herwig/Particles/tbar:NominalMass 173.5
set /Herwig/Particles/h0:NominalMass 125.9
set /Herwig/Particles/b:NominalMass 4.2
set /Herwig/Particles/bbar:NominalMass 4.2
set /Herwig/Particles/W+:Width 2.141
set /Herwig/Particles/W-:Width 2.141
set /Herwig/Particles/Z0:Width 2.4952
set /Herwig/Particles/t:Width 1.4
set /Herwig/Particles/tbar:Width 1.4
set /Herwig/Particles/h0:Width 0.004541
# --------------------------------------------------------------------- #
# 3.) ADDITIONAL SETTINGS

# SHOWER SETTINGS
set /Herwig/Shower/Evolver:IntrinsicPtGaussian 2.2*GeV
# CREATE AND APPLY CUTS
create ThePEG::Cuts /Herwig/Cuts/NoCuts
set /Herwig/EventHandlers/LHEReader:Cuts /Herwig/Cuts/NoCuts
# Switching off MPI
set /Herwig/Shower/ShowerHandler:MPIHandler NULL

# HERE YOU CAN TURN ON/OFF DECAY
# If DecayHandler is set equal to NULL, then set CheckQuark equal to 0, to
# prevent a lot of warnings
#set /Herwig/EventHandlers/LHEHandler:DecayHandler NULL
#set /Herwig/Analysis/Basics:CheckQuark 0
do /Herwig/Particles/h0:SelectDecayModes none
set /Herwig/Particles/h0/h0->mu-,mu+;:OnOff On

#
# Prevent particles from decaying
# Particle names can be found in Herwig++/src/default/mesons.in or similar
#

# pi0
set /Herwig/Particles/pi0:Stable Stable

# 4.) HW++ SETTINGS THAT ARE NOT ALLOWED TO BE TOUCHED BY THE USER

#Boost and reconstruction stuff
set /Herwig/Shower/KinematicsReconstructor:ReconstructionOption General
set /Herwig/Shower/KinematicsReconstructor:InitialInitialBoostOption LongTransBoost

# --------------------------------------------------------------------- #
# 5.) CREATE THE ANALYZER AND SAVE THE RUN (DO NOT MODIFY)

#create MCatNLO::hepfortr hepfortr hepfortr.so
#insert /Herwig/Generators/LHCGenerator:AnalysisHandlers 0 hepfortr

# Useful analysis handlers for HepMC related output
##################################################
# Schematic overview of an event (requires --with-hepmc to be set at configure time
# and the graphviz program 'dot' to produce a plot)
#create MCatNLO:: .so
#insert /Herwig/Generators/LHCGenerator:AnalysisHandlers 0
# insert LHCGenerator:AnalysisHandlers 0 /Herwig/Analysis/Plot
# A HepMC dump file (requires --with-hepmc to be set at configure time)
# insert LHCGenerator:AnalysisHandlers 0 /Herwig/Analysis/HepMCFile
insert /Herwig/Generators/LHCGenerator:AnalysisHandlers 0 /Herwig/Analysis/HepMCFile
set /Herwig/Analysis/HepMCFile:PrintEvent 200000
set /Herwig/Analysis/HepMCFile:Format GenEvent
set /Herwig/Analysis/HepMCFile:Units GeV_mm

saverun MCATNLO_HERWIGPP /Herwig/Generators/LHCGenerator