  We present a novel implementation of a Monte Carlo particle-following code
for solving the distribution function of minority species in fusion plasmas,
called ASCOT5, and verify it using theoretical results for neoclassical
transport. The code has been developed from ground up with an OpenMP-MPI hybrid
paradigm to take full advantage of current and next generation many-core CPUs
with multithreading and SIMD operations. Up to 6-fold increase in performance
is demonstrated compared to a previous version of the code which only utilizes
MPI. The physics model of the code is comprehensively validated against
existing theoretical work, and it is shown to faithfully reproduce neoclassical
diffusion across three different collisionality regimes. In simulations for
realistic tokamak plasmas, including complex non-axisymmetric geometry, ASCOT5
is verified to reproduce results from the previous version ASCOT4.
