Application of DFTB in molecular electronics

Jeffrey R Reimers, reimers@chem.usyd.edu.au1, Gemma C. Solomon,
solomon@chem.usyd.edu.au1, Zheng-Li Cai, zlcai@chem.usyd.edu.au1, Noel S. Hush,
hush_n@chem.usyd.edu.au2, Alessio Gagliardi, gagliard@phys.upb.de3, Thomas
Frauenheim, frauenheim@phys.upb.de4, Alessandro Pecchia5, and Aldo Di Carlo,
dicarlo@ing.uniroma2.it5. (1) School of Chemistry, The University of Sydney,
Sydney, 2006, Australia, (2) School of Molecular and Microbial Biosciences, The
University of Sydney, Sydney, 2006, Australia, (3) Theoretical Physics
Department, University of Bremen, Germany, Vogeliusweg 25.2.1.14, Paderborn,
330918, Germany, (4) Bremen Center for Computational Materials Science, Bremen
University, Bibliothekstrasse 1, Bremen, 28359, Germany, (5) Department of
Electronic Engeneering, University of Rome "Tor Vergata", Rome, Italy

Molecular electronics involves the passing of current between two electrodes
through a single conducting molecule. Calculations in this area require not only
the ability to handle large systems including metal-electrode fragments but also
require accurate positioning of molecular and metallic energy bands and must
treat occupied and virtual orbitals on an equivalent footing. Each of these
requirements presents difficulties for standard DFT calculations, making DFTB an
attractive alternative proposition. We present enhancements to the SCC-DFTB
program that allow it to diagnose and utilize molecular symmetry, increasing
computational speed and accuracy whilst providing important information
concerning molecular orbitals and molecular vibrations. Optimized geometries are
then obtained for molecules sandwiched between gold electrodes, leading to
Green's-function based calculations of steady-state through-molecule electrical
conductivity and incoherent inelastic tunnelling spectroscopy (IETS) arising
from electrical current activation of molecular vibrational modes.