Spin-Symmetry Broken Ground-State of UO$_2$ in DFT+U Approach: The SMC Method

Abstract: It turns out that the ground states of some systems are symmetry-broken states in which some property is not symmetrically distributed. In the case of strongly correlated electron systems, that were studied by the DFT+U method, researchers had shown that the total energy of the system is a multi-minima function of input parameters and one has to single out the ground state out of the couples of minimum-energy states. However, the methods already introduced to determine these local minimum states were not able to predict all such states which may include the "true" ground state. In this work, we introduce a new simple and straight-forward method of SMC to find the GS as well as the meta-stable states of 1k-order anti-ferromagnetic configuration for UO$_2$. Using this method, it is shown that the ground state of UO$_2$ system is a spin-symmetry broken state of the electron spin magnetizations of oxygen atoms. Depending on the way we apply the SMC method, we obtain different numbers of meta-stable states, but the same ground states. The energetic properties, geometric properties, the electronic density distributions, and the electronic polarization density distributions of the ground state and the meta-stable states are shown to be different from each other. These properties also are shown to be sensitive to the magnitude of the initial opposite magnetizations of U1 and U2 atoms in the 1k-order anti-ferromagnetic configuration, but the number of meta-stable states as well as the ground-state properties are insensitive to this magnitude. Using the PBEsol-GGA approximation for the exchange-correlation we obtain the ground-state properties in excellent agreement with experiments.