  The evolution of dense star clusters is followed by direct high-accuracy
N-body simulation. The problem is to first order a gravitational N-body
problem, but stars evolve due to astrophysics and the more massive ones form
black holes or neutron stars as compact remnants at the end of their life.
After including updates of stellar evolution of massive stars and for the
relativistic treatment of black hole binaries we find the growth of
intermediate mass black holes and we show that in star clusters binary black
hole mergers in the so-called pair creation supernova (PSN) gap occur easily.
Such black hole mergers have been recently observed by the LIGO-Virgo-KAGRA
(LVK) collaboration, a network of ground based gravitational wave detectors.
