  Direct detection experiments for light (sub-GeV) dark matter are making
enormous leaps in reaching previously unexplored theory space. The need for
accurate characterizations of target responses has led to a growing interplay
between particle and condensed matter physics. This white paper summarizes
recent progress on direct detection calculations that utilize state-of-the-art
numerical tools in condensed matter physics and effective field theory
techniques. These new results provide the theoretical framework for
interpreting ongoing and planned experiments using electronic and collective
excitations, and for optimizing future searches.
