Localized-density-matrix implementation of time-dependent density-functional theory

The localized single-electron density matrix implementation of time-dependent density-functional theory (TDDFT) was discussed. The excited state properties of atoms and molecules were calculated using the TDDFT. In this regard, the calculations of the absorption spectra of polyacetylene oligomers and linear alkanes by using the TDDFT, were also presented.published_or_final_versio

Dissipative time-dependent quantum transport theory

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Time-dependent density-functional theory/localized density matrix method for dynamic hyperpolarizability

Time-dependent density-functional theory/localized density matrix method (TDDFT/LDM) was developed to calculate the excited state energy, absorption spectrum and dynamic polarizability. In the present work we generalize it to calculate the dynamic hyperpolarizabilities in both time and frequency domains. We show that in the frequency domain the 2n+1 rule can be derived readily and the dynamic hyperpolarizabilities are thus calculated efficiently. Although the time-domain TDDFT/LDM is time consuming, its implementation is straightforward because the evaluation of the derivatives of exchange-correlation potential with respect to electron density is avoided. Moreover, the time-domain method can be used to simulate higher order response which is very difficult to be calculated with the frequency-domain method. © 2007 American Institute of Physics.published_or_final_versio

Linear-scaling time-dependent density-functional theory

A linear-scaling time-dependent density-functional theory is developed to evaluate the optical response of large molecular systems. The two-electron Coulomb integrals are evaluated with the fast multipole method, and the calculation of exchange-correlation quadratures utilizes the locality of exchange-correlation functional within the adiabatic local density approximation and the integral prescreening technique. Instead of many-body wave function, the equation of motion is solved for the reduced single-electron density matrix in the time domain. Based on its "nearsightedness", the reduced density matrix cutoffs are employed to ensure that the computational time scales linearly with the system size. As an illustration, the resulting time-dependent density-functional theory is used to calculate the absorption spectra of linear alkanes, and the linear scaling of computational time versus the system size is clearly demonstrated.published_or_final_versio

Communication: Linear-expansion shooting techniques for accelerating self-consistent field convergence

Based on the corrected Hohenberg-Kohn-Sham total energy density functional [Y. A. Zhang and Y. A. Wang, J. Chem. Phys. 130, 144116 (2009)]10.1063/1. 3104662, we have developed two linear-expansion shooting techniques (LIST)- direct LIST (LISTd) and indirect LIST (LISTi), to accelerate the convergence of self-consistent field (SCF) calculations. Case studies show that overall LISTi is the most robust and efficient algorithm for accelerating SCF convergence, whereas LISTd is advantageous in the early stage of an SCF process. More importantly, LISTi outperforms Pulays direct inversion in the iterative subspace (DIIS) [P. Pulay, J. Comput. Chem. 3, 556 (1982)]10.1002/jcc.540030413 and its two recent improvements, energy-DIIS [K. N. Kudin, G. E. Scuseria, and E. Cancs, J. Chem. Phys. © 2011 American Institute of Physics.published_or_final_versio

A variational approach for dissipative quantum transport in a wide parameter space

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Density matrix based time-dependent density functional theory and the solution of its linear response in real time domain

A density matrix based time-dependent density functional theory is extended in the present work. Chebyshev expansion is introduced to propagate the linear response of the reduced single-electron density matrix upon the application of a time-domain δ -type external potential. The Chebyshev expansion method is more efficient and accurate than the previous fourth-order Runge-Kutta method and removes a numerical divergence problem. The discrete Fourier transformation and filter diagonalization of the first-order dipole moment are implemented to determine the excited state energies. It is found that the filter diagonalization leads to highly accurate values for the excited state energies. Finally, the density matrix based time-dependent density functional is generalized to calculate the energies of singlet-triplet excitations. © 2007 American Institute of Physics.published_or_final_versio

Time-dependent density functional theory based Ehrenfest dynamics

Time-dependent density functional theory based Ehrenfest dynamics with atom-centered basis functions is developed in present work. The equation of motion for electrons is formulated in terms of first-order reduced density matrix and an additional term arises due to the time-dependence of basis functions through their dependence on nuclear coordinates. This time-dependence of basis functions together with the imaginary part of density matrix leads to an additional term for nuclear force. The effects of the two additional terms are examined by studying the dynamics of H 2 and C 2H 4, and it is concluded that the inclusion of these two terms is essential for correct electronic and nuclear dynamics. © 2011 American Institute of Physics.published_or_final_versio

Towards Atomic Level Simulation of Electron Devices Including the Semiconductor-Oxide Interface

We report a milestone in device modeling whereby a planar MOSFET with extremely thin silicon on insulator channel is simulated at the atomic level, including significant parts of the gate and buried oxides explicitly in the simulation domain, in ab initio fashion, i.e without material or geometrical parameters. We use the density-functional-based tight-binding formalism for constructing the device Hamiltonian, and non-equilibrium Green's functions formalism for calculating electron current. Simulations of Si/SiO2 super-cells agree very well with experimentally observed band-structure phenomena in SiO2-confined sub-6 nm thick Si films. Device simulations of ETSOI MOSFET with 3 nm channel length and sub-nm channel thickness also agree well with reported measurements of the transfer characteristics of a similar transistor.published_or_final_versio

Time-dependent density functional theory quantum transport simulation in non-orthogonal basis

Basing on the earlier works on the hierarchical equations of motion for quantum transport, we present in this paper a first principles scheme for time-dependent quantum transport by combining time-dependent density functional theory (TDDFT) and Keldysh's non-equilibrium Green's function formalism. This scheme is beyond the wide band limit approximation and is directly applicable to the case of non-orthogonal basis without the need of basis transformation. The overlap between the basis in the lead and the device region is treated properly by including it in the self-energy and it can be shown that this approach is equivalent to a lead-device orthogonalization. This scheme has been implemented at both TDDFT and density functional tight-binding level. Simulation results are presented to demonstrate our method and comparison with wide band limit approximation is made. Finally, the sparsity of the matrices and computational complexity of this method are analyzed.published_or_final_versio