We present a new, nonarbitrary, internally consistent,
and unambiguous
framework for spin-polarized conceptual density-functional theory
(SP-DFT). We explicitly characterize the convex hull of energy, as
a function of the number of electrons and their spin, as the only
accessible ground states in spin-polarized density functional theory.
Then, we construct continuous linear and quadratic models for the
energy. The nondifferentiable linear model exactly captures the simplicial
geometry of the complex hull about the point of interest and gives exact representations for the conceptual DFT reactivity
indicators. The continuous quadratic energy model is the paraboloid
of maximum curvature, which most tightly encloses the point of interest
and neighboring vertices. The quadratic model is invariant to the
choice of coordinate system (i.e., {N, S} vs {N
α, N
β}) and reduces to a sensible formulation of spin-free
conceptual DFT in the appropriate limit. Using the quadratic model,
we generalize the Parr functions {P
+(r), P
–(r)}
(and their derivatives with respect to number of electrons) to this
new spin-polarized framework, integrating the Parr function concept
into the context of (spin-polarized) conceptual DFT, and extending
it to include higher-order effects.