When using AIMPAC, the flux of this quantity’s gradient is taken to be zero when defining a surface. Perdew et al. proposed a framework for ordering methods of determining this quantity based on chemical accuracy nicknamed “Jacob’s ladder.” In a technique that calculates this quantity, exchange-correlation energy can be estimated by approximations like the GGA and LDA. Because a system’s ground state energy is uniquely dependent on this quantity, (10[1])determining the distribution of this quantity for a chemical is equivalent to “solving” its structure according to the Hohenberg–Kohn theorem. “Functionals” of this quantity are used to model chemicals in DFT. For 10 points, name this quantity that describes a molecule’s negative charge per unit volume. ■END■ (10[1])