Explicit corrections to the gradient expansion for the kinetic energy in one dimension

A mathematical framework is constructed for the sum of the lowest N eigenvalues of a potential. Exactness is illustrated on several model systems (harmonic oscillator, particle in a box, and Poschl-Teller well). Its order-by-order semiclassical expansion reduces to the gradient expansion for slowly-varying densities, but also yields a correction when the system is finite and the spectrum discrete. Some singularities can be avoided when evaluating the correction to the leading term. Explicit corrections to the gradient expansion to the kinetic energy in one dimension are found which, in simple cases, greatly improve accuracy. We discuss the relevance to practical density functional calculations.