An analytical theory of intervalley mixing at semiconductor heterojunctions is presented. Burt's envelope-function representation is used to analyze a pseudopotential Hamiltonian, yielding a simple d-function mixing between G and X electrons and light and heavy holes. This coupling exists even for media differing only by a constant band offset (i.e., with no difference in Bloch functions).[S0031-9007(98)06534-X] PACS numbers: 73.20. Dx, 71.15.Th, 73.61.Ey It is well known in semiconductor physics that bulk effective-mass theory [1] is not valid at an abrupt heterojunction, since the rapid change in potential at the interface causes a mixing of wave functions in different energy bands, and the neglect of such mixing is a key approximation in the development of this theory. It is consequently almost universally believed that a realistic description of the interface can only be achieved numerically, by performing a microscopic supercell calculation. The purpose of this paper is to demonstrate that a careful application of modern envelope-function theory yields a fully analytical description of interface band mixing.The most widely used form of envelope-function theory is Bastard's "envelope-function approximation" (EFA) [2], which openly ignores any interband mixing not found in bulk k ? p theory [3]. Less well known is Burt's theory of the envelope-function representation [4,5], which is an exact representation of the Schrödinger equation, fully capable of describing any effect found in pseudopotential theory. Thus far, the main applications of this theory have been a one-dimensional proof [5] that in long-period superlattices, interface-induced mixing is a small perturbation on the EFA, and the resolution of an ambiguity in the EFA ordering of differential operators [6,7].Unfortunately, the former work [5] is often misconstrued as implying that Burt's theory is no different from the EFA [8][9][10][11]. This interpretation is not warranted, because even small perturbations can have a dramatic impact when they introduce couplings of a qualitatively different nature. In this paper the envelope-function representation is used to analyze an empirical pseudopotential model [12] of the GaAs͞AlAs (001) heterojunction. The result is a simple analytical theory of the interface-induced mixing between G and X electrons [13][14][15][16] and light and heavy holes [17][18][19], in which the coupling takes the form of a finite-width d function whose strength is given directly in terms of pseudopotential form factors. The most striking outcome is that there is no limit in which the EFA is valid for abrupt heterojunctions, since the coupling exists even for identical Bloch functions.I begin by presenting a paraphrased (nonrigorous) version of Burt's theory. Let the microscopic Hamiltonian be H p 2 ͞2m 1 V ͑r͒, and choose as basis functions the complete orthonormal Luttinger-Kohn functions [1] x n ͑r͒ U n ͑r͒e ik?r , where U n ͑r͒ is a periodic Bloch function from some bulk reference crystal (e.g., the virtual crystal Al 0.5 Ga ...