Session K17: Density Functional Theory in Chemical Physics IV

Real-time TDDFT for complex systems and dynamics

Real-time propagation approach to time-dependent density functional theory (RT-TDDFT) is a powerful first-principles method for simulating nonequilibrium electron dynamics. I will start by discussing our method development effort, particularly how massively-parallel computers are used to investigate electron dynamics in large complex systems like DNA solvated in water under ion irradiation. I will then discuss the need for advanced hybrid exchange-correlation approximations in the context of RT-TDDFT and how the unitary transformation of time-dependent orbitals is used to mitigate their prohibitively large cost for condensed matter systems. I will finish the talk by discussing our new effort on using nuclear-electronic orbital (NEO) paradigm for expanding the realm of RT-TDDFT by treating protons quantum-mechanically in heterogeneous systems. Competing kinetics between electronic excitation dynamics and quantum-mechanical proton transfer is discussed as an example of how such new multicomponent DFT formulation can expand the application of RT-TDDFT.

 

    

Exchange-correlation functionals for noncollinear spin density functional theory

We present new semilocal exchange and correlation energy functionals for spin density functional theory (SDFT). These exchange and correlation functionals are directly derived for noncollinear magnetism, in order to preserve gauge invariance, and give rise to non-vanishing exchange correlation torques. We further propose generalization of self-interaction corrections to the non-collinear case, which also give rise to non-vanishing exchange-correlation torques. These functionals are tested for frustrated antiferromagnetic clusters and are shown to perform favorably compared to the far more expensive optimized effective potential for exact exchange. This provides a path forward for functional development in noncollinear SDFT and the ab initio study of magnetic materials in and out of equilibrium.

    

Approximate exchange-correlation energy from model Green's functions

Density functional theory (DFT) and Green’s function (GF) methods are frequently used together to

model realistic band-structure and photoemission spectra. The strength of DFT comes primarily from

its use of the Kohn-Sham scheme to compute groundstate properties, while GF methods make use of

powerful systematic corrections from many-body perturbation theory. The failures and successes of

DFT and GF methods is still not well understood and is a matter of ongoing research. In this work we

derive a Galitskii-Migdal formula to extract the exchange-correlation (XC) energy from exact and

model Green’s functions. Using our formula we separately analyze the XC contributions associated to

different spectral features of Green’s functions. From the exactly solvable Hubbard dimer, we study the

XC energy contributions from exact and GW quasi-particle and satellite peaks. For the dimer we study

the importance of approximating accurate satellite locations and amplitudes. From our XC energy

formula we motivate and derive a local density approximation (LDA) on the Green’s function directly

and produce the well-known XC energy density from DFT. We motivate alternative approximations

using the density functional formalism.   

A DFT analysis of a covalent organic donor-acceptor-radical molecule used for quantum teleportation

Radical ion pairs (RIPs) have been used to demonstrate quantum teleportation in molecular systems for applications in quantum information science. Covalent organic donor-acceptor (D-A) molecules can produce RIPs through photo-induced charge transfer  and an additional radical (R) molecule makes quantum teleportation possible. We present the electronic structure and analyze charge transfer excited states of a recently studied[1] D-A-R molecular system using density functional theory. The distances between donor-acceptor and donor-radical are about 12.9 Å and 21.9 Å, respectively. The excitation energies are calculated using the perturbative delta-SCF method and agree with other conventional excited-state methods and experimental reference values. Charge transfer energies change with solvent polarity, but we find that due to the ionic nature of triad, even low polarity solvents make a significant change in energies. We discuss the spin ordering energies and the Heisenberg exchange coupling parameters for this D-A-R system.

 

[1] Rugg, B.K. et al. Photodriven quantum teleportation of an electron spin state in a covalent donor–acceptor–radical system. Nat. Chem. 11, 981–986 (2019).

 

    

TDDFT-as(p): a series of high accuracy semi-empirical TDDFT model using minimal auxiliary basis.

We report a general strategy to efficiently approximate Time-Dependent Density Functional Theory (TDDFT) calculations of molecular response properties such as UV absorption spectra. This strategy leads to a series of highly accurate semiempirical TDDFT models: TDDFT-as(p). In our benchmarking on 39 small to medium sized molecules, TDDFT-as has an energy error of 0.06 eV, 25% of currently best method sTDDFT with an energy error of 0.24 eV. The advantage of our approach is that we can improve the TDDFT-as model by adding one more p function per atom, which makes the TDDFT-asp model with an energy error of 0.04 eV.  Moreover, TDDFT-as(p) needs a nearly trivial implementation and is naturally ready for existing quantum chemistry softwares such as Turbomole.   

Hubbard clusters as testing grounds for noncollinear spins in (TD)DFT

Commonly used DFT approximations for noncollinear magnetism, most notably the LSDA, are based on reference systems with collinear spins; this neglects exchange-correlation (xc) torque effects. In order to test when these xc torques are important it is helpful to have a fully solvable model to compare to DFT approximations. Finite sized Hubbard models are an ideal testing ground for DFT, but the most common systems in use, the dimer and trimer, are too small to model extended Hubbard interactions, which are necessary for non-trivial exchange torques to arise. We advocate for the use of a half-filled 5-site Hubbard cluster in DFT testing of orbital-dependent xc functionals, including spin-orbit coupling induced geometric frustrations. We find that xc torques make a significant contribution to the noncollinear spin dynamics, especially in the vicinity of boundaries between different phases of frustration.   

Description of the interaction of light and matter using the time-dependent density functional theory

In this talk, I present our recent work on the interaction of light and

matter using the Time-Dependent Density Functional Theory (TDDFT). Two

different versions of the TDDFT calculations will be presented. The

first is the orbital-free TDDFT (OF-TDDFT) which can be used in systems with tens

of thousands of atoms. The OF-TDDFT is coupled to the Maxwell-equations

and the electron density and the electric fields are time propagated

to simulate the interaction of light and matter. We have also worked

on extending the TDDFT to describe the interaction of light and

molecules in cavities using a Quantum Electrodynamic (QED) Hamiltonian.

The QED-TDDFT equations are solved by time propagating the wave function 

on a tensor product of a Fock-space

and real-space grid. Applications for molecules in cavities show the

accuracy of the approach. Examples include the coupling strength and

light frequency dependence of the energies, wave functions, optical

absorption spectra, and Rabi splitting magnitudes in cavities, as well

as a description of high harmonic generation in cavities.   

Modern exact two-component Hamiltonians for relativistic quantum chemistry and physics: Two-electron picture-change corrections made simple

Based on atomic mean-field (amf) SCF quantities, we present two simple, yet computationally efficient and numerically accurate matrix approaches to correct scalar-relativistic and spin-orbit two-electron picture-change corrections (PCs) arising within an exact two-component (X2C) Hamiltonian framework.[1] Both approaches, dubbed amfX2C and eamfX2C, allow us to uniquely tailor PCs to mean-field models, viz. Hartree–Fock or Kohn–Sham DFT, in the latter case also avoiding the need of a point-wise calculation of exchange–correlation PCs. We assess the numerical performance of these Hamiltonians on spinor energies of closed-shell and open-shell molecules, achieving a consistent 10^-5 Hartree accuracy compared to reference four-component (4c) data. Excellent agreements with reference data are also observed for molecular properties sensitive to relativistic effects such as EPR or X-ray absorption energies.[2] We believe that our (e)amfX2C Hamiltonians constitute a fundamental milestone towards a universal and reliable relativistic 2c approach for quantum chemistry and physics, maintaining the accuracy of the parent 4c one at a fraction of its computational cost.   

Quantum Model of Propagating Plexcitons in Two-Dimensional Semiconducting Materials

Hybrid quantum states emerge upon the strong interaction of surface plasmon waves and excitonic species in 2D semiconducting materials. This gives rise to the so-called "plexcitons", which are a special class of polaritonic states that can attain quantum properties that are not possible in either of the decoupled states. In this work, we present a quantum model of the coupling between 2D mobile excitons and mobile surface plasmon polaritons, where both physical states are treated quantum mechanically. Our model shows the emergence of the propagating plexciton states, reflecting a strong coupling between the individual species. The application to a hypothetical device (composed of a gold surface, a spacer, the 2D material, and a capping dielectric) we propose also shows scenarios in which the lower polariton (plexciton) quasiparticle dominates the dynamics of the system upon the injection of surface plasmons to the system. We then discuss opportunities for future work, from both theoretical and experimental perspectives.   

Plasmons and excitons in Pd-doped Ag nanoparticles from an ab initio GW-BSE approach

Combining transition-metal catalyst materials with plasmonic metals can enhance photocatalytic reaction rates, selectivity, and open non-equilibrium reaction paths. These effects are initiated by surface plasmon resonances (SPRs) in the plasmonic metal, and direct electron-hole (e-h) interactions can also significantly alter the landscape of excited states and affect SPR evolution. However, ab initio calculations in realistic metallic nanoparticle systems have mostly either neglected e-h interactions or been limited to very small (<25 atom) clusters that display zero-dimensional-like excitation spectrum due to strong confinement effects. Here, we study Pd-doped Ag nanoparticles of up to 147 atoms with varying dopant levels, including e-h interactions, through the first-principles GW plus Bethe-Salpeter equation (BSE) approach. Applying new low-rank approximations and spectral folding techniques to accelerate these calculations, we can directly obtain excited states of nanoparticles capturing both plasmonic and excitonic effects. For small systems, we observe strong e-h interactions that redshift the spectrum by up to ~2 eV, demonstrating the importance of excitonic effects for spatially confined metallic systems. We also analyze the excitonic and plasmonic character of the excitations as a function of size and doping, and the impact of many-body interactions for reactions involving excited-state potential-energy surfaces.   

Modeling ultrafast photoinduced dynamics of adsorbates on metals with machine learning potentials

I will present theoretical approaches for modeling the dynamics of molecules on metal surfaces induced by short laser pulses.

Modeling is based on a density functional theory derived potential energy surface and the two-temperature model.

The coupling of the adsorbates to the electronic system is modeled in terms of electronic friction and associated stochastic forces that depend on the electronic temperature obtained from the two-temperature model. Using this methodology we have simulated systems such as H/Ru(0001), O2/Ag(110) [1,2], CO/Ru(0001) [3], and CO/Pd(111) [4,5].

We use machine learning to obtain accurate and fast potential energy surfaces that enable long propagation times, realistic coverages, and dynamics in full degrees of freedom.

[1] Phys. Rev. B 93, 014301 (2016) 10.1103/PhysRevB.93.014301

[2] NIM-B 382, 114 (2016) 10.1016/j.nimb.2016.02.051

[3] Phys. Rev. B 94, 165447 (2016) 10.1103/PhysRevB.94.165447

[4] J. Chem. Theory Comput 17, 8, 4648–4659 (2021) 10.1021/acs.jctc.1c00347

[5] Chemical Physics 558, 111518 (2022) 10.1016/j.chemphys.2022.111518