Note to players: Word forms acceptable. Wertheim showed that systems named for this noun have an exact solution for the Percus–Yevick equation. A useful approximation model for ion energies in the primitive model is described by this word and “mean.” Berlin and Kac (“cots”) extended the Ising model to create a model of ferromagnetism (15[1])named for this word. This is the [emphasize] second word of a model described by the Carnahan–Starling equation of state, which includes B-sub-2 equals “four times particle volume” among its analytical solutions for the first three (*) virial coefficients. This is also the first word of a type of function that solves for the angular component of the wavefunction of the hydrogen atom. Those functions denoted “Y-sub-l-super-m” are “harmonics” described by this word. For 10 points, the kinetic theory of gases assumes that all molecules are what idealized “hard” shape with a consistent radius? ■END■