  We present experimental inelastic x-ray scattering (IXS) and ab initio
time-dependent density-functional-theory (TDDFT) studies of YBa2Cu3O7-{\delta}.
The response of the low-lying Ba 5p and Y 4p core electrons is shown to
interact strongly with the Cu 3d and O 2p excitations, with important
consequences on screening. The agreement between IXS and TDDFT results is
excellent, apart from a new type of excitations, mainly related to loosely
bound Ba electrons and significantly affected by correlations. This points to
correlation mechanisms not fully described by TDDFT that might have a role in
giving rise to antiscreening.
