Quantum instanton approach to metastable collective spins

These data underlie the results presented in the paper submitted under the same name [arXiv:2604.15091 (2026)]. K.P. and M.C. acknowledge the financial support of the National Science Centre, Poland, under the project No. 2023/51/D/ST3/01203, and M.E. of the Fond National de la Recherche-FNR, Luxembourg, CORE project NEQPHASETRANS (C24/MS/18933049). The data include: Data used in the plots in the .csv format (file "Plotted data.zip") Wolfram Mathematica notebooks used to derive Hamiltonian H_alpha (file "Derivations of H_alpha.zip") Wolfram Mathematica notebooks used for numerical calculations using QME and instanton approaches, and stability analysis of the MF dynamics (file "Numerics.zip") The code used to plot vector fields in Figs. 2a-c is available at GitHub: https://github.com/mch-ifm/quantum-instantons-in-collective-spin-systems