Code review comment for lp:~maddevelopers/mg5amcnlo/python_syscalc

Hi Olivier,

Ok. I finally had some time to look into this branch. I haven't really tested anything yet, but looking through the changes made in this branch, I noticed that there might be a mistake related to the possible changes of the dynamical scale setting 'dyn'. In particular, it looks like you simply recompute if from the momenta as given in the event file. This is not correct, because this gives different scales to the (MC) subtraction terms in the S and H events, even if those subtraction terms were computed with exactly the same kinematics.

What we do in normal running of the code is the following:

n-body kinematics:
- Born
- Virtual
- (Integrated) FKS Subtraction terms

(n+1)-body kinematics:
- MC subtraction terms (in S and H events)
- Real-emission

See also the calls to 'set_alphaS' in driver_mintMC where the scales are either set using n-body kinematics ('call set_alphaS(p1_cnt(0,1,0))') or (n+1)-body kinematics ('call set_alphaS(p)'), with the subsequent calls to the matrix elements just after those.

I think this needs to be changed, even though in practice this difference is numerically usually small. But to get it 100% correct these changes are needed.

Cheers,
Rik