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, determining the distribution of this quantity for a chemical is equivalent to “solving” its structure according to the Hohenberg–Kohn theorem. (10[1])“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])