A transformation of the virtual Kohn-Sham orbitals is proposed to a set of charge-transfer orbitals (CTOs) adapted to description of CT excitations. The CTO scheme offers a simple estimate of the CT excitation energy with an orbital energy difference. This estimate reproduces well the reference values of the configuration interaction (CI) method in a wide range of donor-acceptor separations in the paradigmatic He - Be complex. CTO-based orbital energy and shape indices are proposed to assess the suitability of the CT description with virtual orbitals of a given basis set. Both indices yield correct trends for the Kohn-Sham and Hartree-Fock orbitals. (C) 2016 Elsevier B.V. All rights reserved.