# FeynArts package fails to load

This is definitely a duplicate of FeynArts not working properly. However, that post dates back 5 years. I am currently on Mathematica 11.0.1.0 with Windows 10 as my OS. I have been trying to load the FeynArts package but to no avail.

I have followed the route of automatic installation prescribed in the FeynCalc wiki(includes an option to install patched version of FeynArts 3.9):

1. Import["https://raw.githubusercontent.com/FeynCalc/feyncalc/master/install.m"]
2. InstallFeynCalc[]


Despite an apparent clean installation, FeynArts doesn't seem to be loaded.

Loading code:

$LoadFeynArts = True; <<FeynCalc;  which returned the output: FeynCalc is already loaded! To reload it, please restart the kernel. Now, when I tried the simplest piece of code one can have via this package: CreateTopologies[1,1->1] Paint[%]  it just returns the input back! How to get this package(FeynArts) working side by side with FeynCalc? • Did you load the package? Or did you just install it? – Szabolcs Mar 8 '18 at 20:18 • @Szabolcs I did install using FeynCalc installer – Subho Mar 8 '18 at 20:22 • You did not answer the question. Did you load the package? When you have a problem, please show all steps needed to reproduce it, not merely some of them. What command did you use to load the package? – Szabolcs Mar 8 '18 at 20:24 • I tried :$LoadFeynArts = True; <<FeynCalc; – Subho Mar 8 '18 at 20:28

## 2 Answers

It worked after implementing a forced patch as mentioned in the wiki https://github.com/FeynCalc/feyncalc/wiki/FeynArts under Patching FeynArts for FeynCalc 9.x and above.

The problem is actually a very simple one: once FeynCalc has been loaded, you cannot reload it or load FeynArts, TARCER, PHI, FeynHelpers or any other add-on without restarting the kernel. So doing something like

<<FeynCalc

...

\$LoadFeynArts=True;
<<FeynCalc


will not work. You need to either issue Quit[] or go to Evaluation -> Quit Kernel -> Local.

I thought that the error message ending with To reload it, please restart the kernel. seems to be pretty clear on on what one should do, but somehow there is still some confusion.

• I was not able to get FeyArts to load even after restarting kernel. I therefore manually extracted the latest tarball for the same and patched it to resolve conflicts with FeynCalc – Subho Mar 12 '18 at 5:08
• In principle, if you choose to install FeynArts while running the automatic installer, it should be patched immediately afterwards. Not sure what went wrong in your case. – vsht Mar 13 '18 at 7:15