Added README.md & original modification time & deleted files

Author: kaiwu-astro

README.md

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+This is Nbody6++GPU - Beijing version, an N-body star cluster simulation code, maintained by Rainer Spurzem (spurzem/at/nao.cas.cn) and team. The starting commits of this repo put all old versions of Nbody6++GPU from Dec 2017 into Git system for version control.
+
+The code is an offspring of Sverre Aarseth's direct N-body codes see www.sverre.com . 
+
+This is the code suitable for parallel and GPU accelerated runs on supercomputers and workstations. 
+
+
+# Documentation
+Manual in `doc/nbody6++_manual.pdf`
+
+
+# Recommended usage:
+## (optional) to enable HDF5
+```bash
+apt-get install libhdf5-openmpi-dev
+apt-get install libhdf5-dev
+```
+## build
+```bash
+ ./configure --with-par=b1m --enable-simd=sse --enable-mcmodel=large
+ make clean ; make -j 
+```
+ It is for up to one million bodies with many initial binaries. The configure script written by 
+ Long Wang has a multitude of further options, check in it or ask.
+
+ Sources are in src/Main/ . Due to urgent bugfixes few routines are later then Dec2020. 
+
+ After make you find the executable and object files in build/  .
+ 
+## Tips
+ The environment variable OMP_NUM_THREADS has to be set to the desired value of 
+ OpenMP threads per MPI process. (Maybe your system has it predefined). I also recommend to set
+ OMP_STACKSIZE=4096M the shell where you run the code.
+
+ It is inefficient (and even more error prone) for particle numbers below about 50k-100k particles (depending on hardware). For smaller N you are advised to use Nbody6 and Nbody6GPU for single node/process.
+ 
+ It is recommended to provide a dat.10 file in N-body input format (see manual). Such file can be produced by other prograns, like mcluster.
+
+
+# Seleted References:
+ - https://ui.adsabs.harvard.edu/abs/1999PASP..111.1333A/abstract (Aarseth: NBODY1 to NBODY6)
+ - https://ui.adsabs.harvard.edu/abs/1999JCoAM.109..407S/abstract (Spurzem on NBODY6++)
+ - https://ui.adsabs.harvard.edu/abs/2005MNRAS.363..293H/abstract (Hurley+ on SSE/BSE, earlier references therein)
+ - https://ui.adsabs.harvard.edu/abs/2012MNRAS.424..545N/abstract (Nitadori+: NBODY6GPU)
+ - https://ui.adsabs.harvard.edu/abs/2015MNRAS.450.4070W/abstract (Wang+: NBODY6++GPU)
+ - https://ui.adsabs.harvard.edu/abs/2021arXiv210508067K/abstract (Kamlah+: More on current stellar evol.)
+
+
+# Known Problems:
+   1. For systems with more than one GPU on one node the association of MPI rank id and GPU bus id is not 
+      well defined, will be improved in next version.
+   2. Runs with a million or more bodies and huge numbers of binaries (5% or more) use extreme amounts of
+      computing time for the KS binaries (much much more than should be expected). We work on this. 
+   3. On some systems heap and stack management when using OpenMP and MPI together seem to produce very
+      strange errors and segmentation faults. The exact reason is not known; we work on this.
+
+
+# Disclaimer
+ This code and the documentation is given without warranty, hopefully it is helpful. All may contain errors.