  Ultracold neutrons (UCN) with kinetic energies up to 300 neV can be stored in
material or magnetic confinements for hundreds of seconds. This makes them a
very useful tool for probing fundamental symmetries of nature, by searching for
charge-parity violation by a neutron electric dipole moment, and yielding
important parameters for Big Bang nucleosynthesis, e.g. in neutron-lifetime
measurements. Further increasing the intensity of UCN sources is crucial for
next-generation experiments. Advanced Monte Carlo (MC) simulation codes are
important in optimization of neutron optics of UCN sources and of experiments,
but also in estimation of systematic effects, and in bench-marking of analysis
codes. Here we will give a short overview of recent MC simulation activities in
this field.
