Pseudopotential description of rare earths in oxides: The case of ${\mathrm{Er}}_2{\mathrm{Si}}_2{\mathrm{O}}_7$

The applicability of ultrasoft pseudopotentials to the problem of rare-earth incorporation in silicates is investigated using the compound ${\mathrm{Er}}_2{\mathrm{Si}}_2{\mathrm{O}}_7$ as a test case. It is found that density-functional theory within the generalized gradient approximation provides a good description of the structural parameters, when treating the Er $4f$ states as a partially occupied core shell. The density of states and the distribution of electronic charge are analyzed, and it is concluded that the presence of Er tends to increase the covalency of neighboring Si-O bonds.