Hi MG5_aMC team, We have observed in the ATLAS software in v2.6.1 and have reproduced with a standalone setup using v2.6.6 that the first weight in the LHE file is not the nominal weight. E.g. with: [--mur=0.5,1,2', '--muf=0.5,1,2', '--pdf=errorset'] = systematics_arguments We see something like the following in the LHEF: MUR=0.5 MUF=0.5 MUR=0.5 MUF=0.5 dyn_scale_choice=sum pt MUR=0.5 MUF=0.5 dyn_scale_choice=HT ... 4 1 +4.5600700e-02 1.74785200e+02 7.54677100e-03 1.07475900e-01 21 -1 0 0 501 502 +0.0000000000e+00 +0.0000000000e+00 +9.0884757352e+01 9.0884757352e+01 0.0000000000e+00 0.0000e+00 1.0000e+00 21 -1 0 0 502 503 -0.0000000000e+00 -0.0000000000e+00 -3.7554094781e+02 3.7554094781e+02 0.0000000000e+00 0.0000e+00 1.0000e+00 6 1 1 2 501 0 +1.5198609831e+01 -1.9745396748e+01 -6.6786755993e+01 1.8711052672e+02 1.7300000000e+02 0.0000e+00 1.0000e+00 -6 1 1 2 0 503 -1.5198609831e+01 +1.9745396748e+01 -2.1786943447e+02 2.7931517844e+02 1.7300000000e+02 0.0000e+00 1.0000e+00 2 0.17478524E+03 0 1 21 0.13982270E-01 0.17478524E+03 1 21 0.57775531E-01 0.17478524E+03 0.14259007E+05 +5.7582231e-02 +7.0903054e-02 +4.5600700e-02 ... This is pretty dangerous since it puts one of the scale variations as the first weight which is the default weight for the entire analysis chain... Also, I believe it breaks the LHEFv3 convention: https://arxiv.org/pdf/1405.1067.pdf (p164) where it says: "syntax should follow this example: This is the original event weight ". Could you check this and in future ensure the nominal weight is at "id=1"? Best, Josh

When	Who	Action	New State
2019-07-03	Olivier Mattelaer	Comment	Open
2019-07-03	Josh McFayden	Comment	Open
2019-07-04	Olivier Mattelaer	Answer	Answered

MadGraph5_aMC@NLO

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