  We study the out-of-equilibrium production of non-minimally coupled
self-interacting scalar dark matter during reheating using classical lattice
simulations. The outcomes of the classical simulations are in qualitative
agreement with the previous results obtained using the quantum 2PI approach in
the Hartree truncation. In particular, the novel non-linear resonance found in
the 2PI Hartee study is present also in the classical lattice simulations and
can dominate the final dark matter yield. For the parameters considered, the
difference in final value of the scalar two-point function between the two
approaches is a factor of O(1).
