  Utilizing the PYTHIA8 Angantyr model, which incorporates the multiple-parton
interactions (MPI) based color reconnection (CR) mechanism, we study the
relative neutron density fluctuation and neutron-proton correlation in Au+Au
collisions at $\sqrt{s_\text{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and
200 GeV. In this study, we have not only delved into the dependence of these
two remarkable observations on rapidity, centrality, and energy, but also
presented an analysis of their interplay with the MPI and CR. Our results have
shown that the light nuclei yield ratio of proton, deuteron, and triton,
expressed by the elegant expression $N_tN_p/N_d^2$, remains unchanged even as
the rapidity coverage and collision centrality increase. Interestingly, we have
also revealed that the effect of CR is entirely dependent on the presence of
MPI; CR has no impact on the yield ratio if MPI is off. Our findings further
demonstrate that the light nuclei yield ratio experiences a slight increase
with increasing collision energy as predicted by the PYTHIA8 Angantyr model,
but it cannot describe the non-monotonic trend observed by the STAR experiment.
Based on the Angantyr model simulation results, it is essential not to overlook
the correlation between neutron and proton fluctuations. The Angantyr model is
a good baseline for studying collisions in the absence of a Quark-Gluon Plasma
(QGP) system, given its lack of flow and jet quenching.
