document how to run with an external generator

README.md

@@ -2,7 +2,8 @@
 This repository shows basic examples of how to run the software stack and perform various tasks.
 
 ## [Creating Monte Carlo samples interactively](scripts/create_mc.sh)
-To creating a few events with a build-in generator run
+### Internal generators
+For creating a few events with a build-in generator run
 ```
 runsim -VERSION va0u -LABO CERN -NRUN 1000 -EBEAM 45.625 -igen qqps -NEVMAX 10
 
@@ -25,6 +26,15 @@ Created files:
 
 Analysis can be run as well on the full DST output in which case a bunch of packages will be rerun.
 
+### External generators
+In this case the generator is run externally and the output is written to a file in lund format. This can then
+be passed through the detector simulation with
+```
+runsim -VERSION va0u -LABO CERN -NRUN 1000 -EBEAM 45.625 -gext generated.lund -NEVMAX 10
+
+```
+Old executables for generators can be found in /cvmfs/delphi.cern.ch/mc-production/generators/pgf77_glibc2.2
+
 ## [Creating Monte Carlo samples on the HTCondor batch farm at CERN](htcondor/create_mc.sub)
 To run the same job on the HTCondor batch farm, you need to follow these steps
 

htcondor/create_mc.sub

+executable            = create_mc.sh
+arguments             = $(ClusterID) $(ProcId)
+output                = output/create_mc.$(ClusterId).$(ProcId).out
+error                 = error/create_mc.$(ClusterId).$(ProcId).err
+log                   = log/create_mc.$(ClusterId).log
+requirements = (OpSysAndVer =?= "CentOS7")
+queue