title: No need for external orthogonality in subsystem density-functional theory.
abstract: Recent reports on the necessity of using externally orthogonal orbitals in subsystem density-functional theory (SDFT) [Annu. Rep. Comput. Chem., 8, 2012, 53; J. Phys. Chem. A, 118, 2014, 9182] are re-investigated. We show that in the basis-set limit, supermolecular Kohn-Sham-DFT (KS-DFT) densities can exactly be represented as a sum of subsystem densities, even if the subsystem orbitals are not externally orthogonal. This is illustrated using both an analytical example and in basis-set free numerical calculations for an atomic test case. We further show that even with finite basis sets, SDFT calculations using accurate reconstructed potentials can closely approach the supermolecular KS-DFT density, and that the deviations between SDFT and KS-DFT decrease as the basis-set limit is approached. Our results demonstrate that formally, there is no need to enforce external orthogonality in SDFT, even though this might be a useful strategy when developing projection-based DFT embedding schemes.
authors.affiliation: Theoretische Organische Chemie, Organisch-Chemisches Institut and Center for Multiscale Theory and Computation, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149 Münster, Germany. j.neugebauer@uni-muenster.de.
authors.forename: Jan P
authors.initials: JP
authors.lastname: Unsleber
authors.affiliation: Theoretische Organische Chemie, Organisch-Chemisches Institut and Center for Multiscale Theory and Computation, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149 Münster, Germany. j.neugebauer@uni-muenster.de.
authors.forename: Johannes
authors.initials: J
authors.lastname: Neugebauer
authors.affiliation: Institute of Physical and Theoretical Chemistry, TU Braunschweig, Hans-Sommer-Straße 10, 38106 Braunschweig, Germany. c.jacob@tu-braunschweig.de.
authors.forename: Christoph R
authors.initials: CR
authors.lastname: Jacob