About TDAP

TDAP stands for "Time Dependent Ab-initio Package", which is a suite of computer software targeting for dynamic simulations of electron-ion system from first principles. It employs a real time real space implementation of time-dependent density functional theory based on numerical atomic basis sets.

The Born-Oppenheimer approximation, which assumes that the motion of the nuclei and electrons in a molecule or solid can be separated, is the most fundamental hypothesis in quantum physics and chemistry. However, in many situations, such as chemical or biological processes involving electron or proton transfer with significant tunneling or nonadiabatic effects, zero point motion in a chemical bond containing available energy smaller than that predicted by the potential depth, and the continuous rearrangement of a complex network of hydrogen bonds in water (inherently a quantum mechanical phenomenon), the Born-Oppenheimer approximation may often fail due to neglecting quantum mechanical electron-nucleus correlation effects. In such processes, the molecular system owns enough energy to explore the unusual regions of the configuration space, the adiabatic potential energy surface (PES) driving the time evolution of the system branches, and the nuclear wave packet splits among the manifolds of possible states.

The theoretical treatment of the time-dependent nonadiabatic phenomena is a formidable challenge at many levels, from the description of the excited states to the time propagation of the corresponding physical properties. Given that the full quantum mechanical solution of such problems for large systems is out of question, several semi-classical approaches have been developed in the last half century to tackle the problem.

TDAP is a real-time ab initio approach for electron-nucleus dynamic simulations beyond the Born-Oppenheimer approximation, which follows Ehrenfest dynamics and represents a mean-field theory of the mixed quantum-classical system, with forces on the nuclei are averaged over many possible adiabatic electronic states induced by nuclei motion.

TDAP employs local atomic basis sets and real-time propagation of wave functions for solving the time-dependent Kohn-Sham (TDKS) equations, which endows it several advantages over available conventional methods:

  • The adoption of overwhelmingly efficient atomic orbital basis sets, which are small in size and fast in performance, enables simulations of either periodic system or a finite-sized supercell with large vacuum space without heavy calculation cost while maintaining relatively high accuracy.

  • Real time excited state trajectories are achieved with many-electron density self-consistently propagating at every electronic and nuclear steps and forces calculated from mean-field theory, offering a direct microscopic picture on the ultrafast dynamics of electrons and nuclei upon photo-excitation.

  • Relatively high efficiency for parallelization can be achieved because the occupied molecular orbitals are propagated independently at a time and can be distributed evenly over several processors with little mutual communication.

  • Photo-absorption spectra and polarizability of the computed systems can be calculated within the same scheme. Nonlinear effects can also be treated precisely.

TDAP behaves well in treating dynamic processes such as interface electron injection, electron-hole recombination and charge transfer induced chemical reactions, where a single path dominates in the reaction dynamics. However, in mean-field regime, Ehrenfest dynamics describes nuclear paths using a single averaged trajectory even when the nuclear wavefunction has broken up into distinct parts. Therefore, the approach fails to deal with situations where multiple paths are involved in the excited states, especially when state-specific nuclear trajectories are of interest. It also lacks a detailed balance for quantum electronic states For alternative strategies where trajectories other than Ehrenfest dynamics are needed to model nonadiabatic processes, the readers are referred to methods which explicitly include electronic transitions such as trajectory surface hopping.

Major features of TDAP include

  • Prediction of optical spectrum
  • To calculate optical properties of a molecule or material under investigation, one monitors the dynamic evolution of the electric dipole moment of the system after perturbation by an external field based on real-time TDDFT simulations. The optical absorbance can be obtained by fourier transform of the time evolution of dipole moments. Nonlinear optical response of molecules and materials can also be studied by explicitly looking at its field dependence.

  • Ultrafast electron injection
  • Ultrafast electron and hole dynamics upon light excitation in molecular and semiconducting systems is essentially a key process in many types of photovoltaic and optoelectronic devices. This process is usually coupled to local or periodic atomic vibrations during carrier transport and relaxation processes, so both the electronic and nuclear degrees of freedom need to be considered. TDAP can be used to simulate interface electron injection dynamics.

  • Excited state molecular dynamics
  • All experimental probes activate the system to excited states for detection. However, a full description of excited states, at the atomic [molecular vibrations] and electronic [absorption and emission] level, especially the coupled ion-electron motions under strong field [quantum control] is highly challenging, but can now be tested within TDAP method.

    The following papers describe the method and practical implementation of the TDAP package:

    • C. Lian, M.X. Guan, S.Q. Hu, J. Zhang, S. Meng. Photoexcitation in solids: First-principles quantum simulations by real-time TDDFT. Adv. Theo. Simul. 1, 1800055 (2018).

    • W. Ma, J. Zhang, L. Yan, Y. Jiao, Y. Gao, S. Meng. Recent progresses in real-time local-basis implementation of time dependent density functional theory for electron-nucleus dynamics. Comp. Mater. Sci. 112, 478 (2016).

    • J. Ren, E. Kaxiras, S. Meng. Optical properties of clusters and molecules from real-time time-dependent density functional theory using a self-consistent field. Molecular Physics 108, 1829 (2010).

    • S. Meng, E. Kaxiras. Local basis-set and real time implementation of time-dependent density-functional theory for excited state dynamics simulations. J. Chem. Phys. 129, 054110 (2008).

    2015 The TDAP team