Abstract
Based on the non-equilibrium Green's function (NEGF) and time dependent density functional theory (TDDFT), we propose a formalism to study the time dependent transport behavior of molecular device from first principles. While this approach is equivalent to the time-dependent wave function approach within TDDFT, it has the advantage that the scattering states and bound states are treating on the equal footing. We have applied our formalism to calculate the transient current of two molecular devices Al-1, 4-dimethylbenzene-Al and Al-Benzene-Al from first principles. In the calculation, no approximations including wideband limit have been made in the single electron NEGF theory and the adiabatic local density approximation was used within TDDFT. It is known that when the wideband limit is abandoned, the boundary condition of the transport problem is non-Markovian resulting a memory term in the effective Hamiltonian of the scattering region. To overcome the computational complexity due to the memory term, we have employed a fast algorithm to speed up the calculation and reduced the CPU time from scaling N3 to N2log22(N) for the step like pulse, where N is the number of time step in the time evolution of Green's function. To ensure the accuracy of our method, we have done a benchmark transient calculation on an atomic junction using a time-dependent wave function approach within TDDFT in momentum space which agrees very well with the result from our method.