A self-consistent tight binding model for hydrocarbon systems: application to quantum transport simulation

A self-consistent environment-dependent (SC-ED) tight binding (TB) method for hydrocarbons that was developed for quantum transport simulations is presented. The method builds on a non-self-consistent environment-dependent TB model for carbon (Tang et al 1996 Phys. Rev. B 53 979) with parameters added to describe hydrocarbon bonds and to account for self-consistent charge transfer. The SC-EDTB model assumes an orthogonal basis set. Orthogonality is a key element for adapting the SC-EDTB scheme to transport problems because it substantially increases the efficiency of the Newton-Raphson algorithm used to accelerate self-consistency convergence under nonequilibrium conditions. Compared to most existing TB schemes the SC-EDTB scheme is distinctive in two respects. First, self-consistency is added through the exact evaluation of Hartree and linear expansion of exchange integrals. All Hamiltonian elements belonging to the same atom are affected by charge transfer, not just the diagonal elements. The second distinction is the choice of SC-EDTB parameters; they were fitted to Mulliken populations and eigenvalue spectra rather than energies or elastic properties. The former are directly related to the conductivity and potential profile, which are essential for transport simulation. No two-centre repulsive term parametrization was performed. The functionality of the method is exemplified by computing I-V curves, nonequilibrium potential profiles and current density for a resonant tunnelling device.