Demazure Crystals, Kirillov-Reshetikhin Crystals, and the Energy Function

It has previously been shown that, at least for non-exceptional Kac–Moody Lie algebras, there is a close connection between Demazure crystals and tensor products of Kirillov–Reshetikhin crystals. In particular, certain Demazure crystals are isomorphic as classical crystals to tensor products of Kirillov–Reshetikhin crystals via a canonically chosen isomorphism. Here we show that this isomorphism intertwines the natural affine grading on Demazure crystals with a combinatorially defined energy function. As a consequence, we obtain a formula of the Demazure character in terms of the energy function, which has applications to Macdonald polynomials and q-deformed Whittaker functions.