7 research outputs found
Nonadiabatic Dynamics for Electrons at Second-Order: Real-Time TDDFT and OSCF2
We
develop a new model to simulate nonradiative relaxation and
dephasing by combining real-time Hartree–Fock and density functional
theory (DFT) with our recent open-systems theory of electronic dynamics.
The approach has some key advantages: it has been systematically derived
and properly relaxes noninteracting electrons to a Fermi–Dirac
distribution. This paper combines the new dissipation theory with
an atomistic, all-electron quantum chemistry code and an atom-centered
model of the thermal environment. The environment is represented nonempirically
and is dependent on molecular structure in a nonlocal way. A production
quality, <i>O</i>(<i>N</i><sup>3</sup>) closed-shell
implementation of our theory applicable to realistic molecular systems
is presented, including timing information. This scaling implies that
the added cost of our nonadiabatic relaxation model, time-dependent
open self-consistent field at second order (OSCF2), is computationally
inexpensive, relative to adiabatic propagation of real-time time-dependent
Hartree–Fock (TDHF) or time-dependent density functional theory
(TDDFT). Details of the implementation and numerical algorithm, including
factorization and efficiency, are discussed. We demonstrate that OSCF2
approaches the stationary self-consistent field (SCF) ground state
when the gap is large relative to <i>k</i><sub><i>b</i></sub><i>T</i>. The code is used to calculate linear-response
spectra including the effects of bath dynamics. Finally, we show how
our theory of finite-temperature relaxation can be used to correct
ground-state DFT calculations
Nonadiabatic Dynamics for Electrons at Second-Order: Real-Time TDDFT and OSCF2
We
develop a new model to simulate nonradiative relaxation and
dephasing by combining real-time Hartree–Fock and density functional
theory (DFT) with our recent open-systems theory of electronic dynamics.
The approach has some key advantages: it has been systematically derived
and properly relaxes noninteracting electrons to a Fermi–Dirac
distribution. This paper combines the new dissipation theory with
an atomistic, all-electron quantum chemistry code and an atom-centered
model of the thermal environment. The environment is represented nonempirically
and is dependent on molecular structure in a nonlocal way. A production
quality, <i>O</i>(<i>N</i><sup>3</sup>) closed-shell
implementation of our theory applicable to realistic molecular systems
is presented, including timing information. This scaling implies that
the added cost of our nonadiabatic relaxation model, time-dependent
open self-consistent field at second order (OSCF2), is computationally
inexpensive, relative to adiabatic propagation of real-time time-dependent
Hartree–Fock (TDHF) or time-dependent density functional theory
(TDDFT). Details of the implementation and numerical algorithm, including
factorization and efficiency, are discussed. We demonstrate that OSCF2
approaches the stationary self-consistent field (SCF) ground state
when the gap is large relative to <i>k</i><sub><i>b</i></sub><i>T</i>. The code is used to calculate linear-response
spectra including the effects of bath dynamics. Finally, we show how
our theory of finite-temperature relaxation can be used to correct
ground-state DFT calculations
Nonradiative Relaxation in Real-Time Electronic Dynamics OSCF2: Organolead Triiodide Perovskite
We
apply our recently developed nonequilibrium real-time time-dependent
density functional theory (OSCF2) to investigate the transient spectrum
and relaxation dynamics of the tetragonal structure of methylammonium
lead triiodide perovskite (MAPbI<sub>3</sub>). We obtain an estimate
of the interband relaxation kinetics and identify multiple ultrafast
cooling channels for hot electrons and hot holes that largely corroborate
the dual valence–dual conduction model. The computed relaxation
rates and absorption spectra are in good agreement with the existing
experimental data. We present the first <i>ab initio</i> simulations of the perovskite transient absorption (TA) spectrum,
substantiating the assignment of induced bleaches and absorptions
including a Pauli-bleach signal. This paper validates both OSCF2 as
a good qualitative model of electronic dynamics, and the dominant
interpretation of the TA spectrum of this material
Accelerating Realtime TDDFT with Block-Orthogonalized Manby–Miller Embedding Theory
Realtime
time-dependent density-functional theory (RT-TDDFT) is
one of the most practical techniques available to simulate electronic
dynamics of molecules and materials. Promising applications of RT-TDDFT
to study nonlinear spectra and energy transport demand simulations
of large solvated systems over long time scales, which are computationally
quite costly. In this paper, we apply an embedding technique developed
for ground-state SCF methods by Manby and Miller to accelerate realtime
TDDFT. We assess the accuracy and speed of these approximations by
studying the absorption spectra of solvated and covalently split chromophores.
Our embedding approach is also compared with less accurate, less costly
QM/MM charge embeddings. We find that by mixing levels of detail the
embedded mean-field theory scheme is a simple, accurate, and effective
way to accelerate RT-TDDFT simulations
Intrinsic Bond Energies from a Bonds-in-Molecules Neural Network
Neural networks are being used to
make new types of empirical chemical
models as inexpensive as force fields, but with accuracy similar to
the ab initio methods used to build them. In this work, we present
a neural network that predicts the energies of molecules as a sum
of intrinsic bond energies. The network learns the total energies
of the popular GDB9 database to a competitive MAE of 0.94 kcal/mol
on molecules outside of its training set, is naturally linearly scaling,
and applicable to molecules consisting of thousands of bonds. More
importantly, it gives chemical insight into the relative strengths
of bonds as a function of their molecular environment, despite only
being trained on total energy information. We show that the network
makes predictions of relative bond strengths in good agreement with
measured trends and human predictions. A Bonds-in-Molecules Neural
Network (BIM-NN) learns heuristic relative bond strengths like expert
synthetic chemists, and compares well with ab initio bond order measures
such as NBO analysis
Black-Box, Real-Time Simulations of Transient Absorption Spectroscopy
We introduce an atomistic, all-electron,
black-box electronic structure
code to simulate transient absorption (TA) spectra and apply it to
simulate pyrazole and a GFP-chromophore derivative. The method is
an application of OSCF2, our dissipative extension of time-dependent
density functional theory. We compare our simulated spectra directly
with recent ultrafast spectroscopic experiments. We identify features
in the TA spectra to Pauli-blocking, which may be missed without a
first-principles model. An important ingredient in this method is
the stationary-TDDFT correction scheme recently put forward by Fischer,
Govind, and Cramer that allows us to overcome a limitation of adiabatic
TDDFT. We demonstrate that OSCF2 is able to reproduce the energies
of bleaches and induced absorptions as well as the decay of the transient
spectrum with only the molecular structure as input
Origin of the Size-Dependent Stokes Shift in CsPbBr<sub>3</sub> Perovskite Nanocrystals
The
origin of the size-dependent Stokes shift in CsPbBr<sub>3</sub> nanocrystals
(NCs) is explained for the first time. Stokes shifts
range from 82 to 20 meV for NCs with effective edge lengths varying
from ∼4 to 13 nm. We show that the Stokes shift is intrinsic
to the NC electronic structure and does not arise from extrinsic effects
such as residual ensemble size distributions, impurities, or solvent-related
effects. The origin of the Stokes shift is elucidated via first-principles
calculations. Corresponding theoretical modeling of the CsPbBr<sub>3</sub> NC density of states and band structure reveals the existence
of an intrinsic confined hole state 260 to 70 meV above the valence
band edge state for NCs with edge lengths from ∼2 to 5 nm.
A size-dependent Stokes shift is therefore predicted and is in quantitative
agreement with the experimental data. Comparison between bulk and
NC calculations shows that the confined hole state is exclusive to
NCs. At a broader level, the distinction between absorbing and emitting
states in CsPbBr<sub>3</sub> is likely a general feature of other
halide perovskite NCs and can be tuned via NC size to enhance applications
involving these materials