Merge pull request #3 from JETSCAPE/ip-glasma-docs

updated IP-Glasma instructions

Author: latessa

external_packages/README.md

@@ -137,3 +137,35 @@ To run JetScape test with SMASH:
 
 Currently the iSS sampler performs resonance decays after sampling.
 For reasonable physics with SMASH these decays should be switched off.
+
+### Installing and Compiling X-SCAPE with IP-Glasma
+
+The use of IP-Glasma with the current version of X-SCAPE is not fully integrated.  A workaround is provided here and applies to using the **jetscape/base:stable** Docker image.
+
+```
+cd ${JETSCAPE_DIR}/external_packages
+./get_ipglasma.sh
+```
+After the IP-Glasma package downloads, replace the file **${JETSCAPE_DIR}/external_packages/ipglasma/CMakeModules/FindFFTW.cmake** with this alternative FindFFTW.cmake found here:
+
+[Alternative FindFFTW.cmake File](https://git.jinr.ru/nica/bmnroot/-/blob/9fb98e26eb3e27fe379d3a61bad5d1567665bd81/cmake/modules/FindFFTW.cmake)
+
+Then create a file called fftw3.h in the **${JETSCAPE_DIR}/external_packages/ipglasma/src** folder that includes the contents of the file linked here:
+
+[fftw3.h File](https://github.com/FFTW/fftw3/blob/master/api/fftw3.h)
+
+Create a build folder and cd into it.
+```
+cd ${JETSCAPE_DIR}
+mkdir build
+cd build
+```
+
+Include the fftw library paths as part of your cmake command:
+
+```
+cmake .. -DCMAKE_CXX_STANDARD=14 -DUSE_ROOT=ON -DUSE_MUSIC=ON -DUSE_ISS=ON -DUSE_FREESTREAM=ON -DUSE_SMASH=ON -DUSE_IPGlasma=ON -DFFTW_INCLUDE_DIR=/usr/lib/x86_64-linux-gnu/libfftw3.so.3.5.8 -DFFTW_LIBRARY=/usr/lib/x86_64-linux-gnu/libfftw3.so.3.5.8
+
+make -j4     # Builds using 4 cores; adapt as appropriate
+```
+If you're not compiling all the external modules, you don't have to turn them on.