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We present results of a detailed theoretical study of the electronic,magnetic, and structural properties of the chalcogenide parent system FeSeusing a fully charge self-consistent implementation of the density functionaltheory plus dynamical mean-field theory (DFT+DMFT) method. In particular, wepredict a remarkable change of the electronic structure of FeSe which isaccompanied by a complete reconstruction of the Fermi surface topology(Lifshitz transition) upon a moderate expansion of the lattice volume. Thephase transition results in a change of the in-plane magnetic nesting wavevector from $(\pi,\pi)$ to $(\pi,0)$ and is associated with a transition fromitinerant to orbital-selective localized magnetic moments. We attribute thisbehavior to a correlation-induced shift of the van Hove singularity of the Fe$t_{2}$ bands at the M-point across the Fermi level. Our results reveal astrong orbital-selective renormalization of the effective mass $m^*/m$ of theFe $3d$ electrons upon expansion. The largest effect occurs in the Fe $xy$orbital, which gives rise to a non-Fermi-liquid-like behavior above thetransition. The behavior of the momentum-resolved magnetic susceptibility$\chi({\bf q})$ demonstrates that magnetic correlations are also characterizedby a pronounced orbital selectivity, suggesting a spin-fluctuation origin ofthe nematic phase of paramagnetic FeSe. We conjecture that the anomalousbehavior of FeSe upon expansion is associated with the proximity of the Fe$t_{2}$ van Hove singularity to the Fermi level and the sensitive dependence ofits position on external conditions.

Effect of electron correlations on the electronic structure and phase stability of FeSe upon lattice expansion