Applications Of Time-dependent Density Functional Theory In Ultrafast Dynamics

With the rapid development of supercomputing technology,first-principles computing methods based on density functional theory(DFT)have been widely used in condensed matter physics,quantum chemistry,materials science,and biomedicine.However,the traditional density functional theory is limited to only deal with the ground state properties of the system,and cannot describe the dynamics of the excited state well.Therefore,it is necessary to realize the calculation method based on the time-dependent density functional theory(TDDFT).Based on the first-principles software package ABACUS developed by our group based on numerical orbital basis sets,we have realized the real-time evolution time-dependent density functional theory(rt-TDDFT)program,and by constructing the light field and changing the electron occupancy and other forms to realize the excitation of electrons in the system,our computational method and program can effectively deal with the excited state properties generated by the interaction of light and matter.On the other hand,the advancement of high-energy ultrafast laser technology has made the study of excited state dynamics move towards the femtosecond field,and various novel physical phenomena arising from the interaction between laser and quantum materials have also attracted our attention.Driven by a light field of a specific frequency,the photomolecular switch can realize the mutual conversion of isomerized molecules of different configurations.For example,the trans-cis isomerization of azobenzene molecules can realize solar energy storage and optoelectronic switching.However,the kinetic mechanism of the isomerization process is still controversial.In addition,non-centrosymmetric crystals represented by two-dimensional tungsten disulfide will have a second-order optical response under illumination,resulting in a shift current.The physical origin and temperature effects of this current also lack sufficient theoretical studies.In this paper,using the developed rt-TDDFT procedure,we investigate the isomerization process of azobenzene and bridged azobenzene molecules and simulate the generation and decay of shift currents in two-dimensional tungsten disulfide crystals,illustrating our computational approach can be effectively applied to the study of ultrafast excited state dynamics.The main research results obtained in this paper are as follows:1.Combined with the mean field theory,based on the existing ground state DFT calculation in ABACUS,the time-dependent density functional theory program with real-time evolution is realized.Compared with other programs,we can simulate the excited state dynamic process of large-scale systems for a long time.The cost of computation is reduced while maintaining accuracy.2.Based on the time-dependent density functional theory program developed by ourselves,we simulated the isomerization process of azobenzene molecules,and concluded that the mechanism of photoisomerization conforms to the mechanism of inversion-assisted rotation.And we further analyzed the coupling of electronic states and electro-phonons,and studied the regulation of the optical field in the process of isomerization.In the end,we similarly studied the bridged azobenzene molecule,and concluded that the photoisomerization process of this molecule is accompanied by the phenomenon of cross-space charge transfer.3.We use the real-time time-dependent density functional theory to study the properties of the two-dimensional tungsten disulfide shift current.We give the calculation method of the shift current,and calculate the dependence of the shift current conductance with the frequency of the optical field.The generation and decay curves of the shift current are simulated,and the effect of temperature on the shift current is also discussed.