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BSEdyn.tex
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BSEdyn.tex
@ -736,7 +736,7 @@ Although it might be reduced to $\order*{\Norb^4}$ operations with standard reso
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\section{Computational details}
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\label{sec:compdet}
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%%%%%%%%%%%%%%%%%%%%%%%%
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All systems under investigation have closed-shell singlet ground states.
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All systems under investigation have a closed-shell singlet ground state.
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We then adopt a restricted formalism throughout this work.
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The $GW$ calculations performed to obtain the screened Coulomb operator and the quasiparticle energies are done using a (restricted) HF starting point.
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Perturbative $GW$ (or {\GOWO}) \cite{Hybertsen_1985a, Hybertsen_1986} quasiparticle energies are employed as starting points to compute the BSE neutral excitations.
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@ -744,7 +744,9 @@ These quasiparticle energies are obtained by linearizing the frequency-dependent
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Further details about our implementation of {\GOWO} can be found in Refs.~\onlinecite{Loos_2018b,Veril_2018}.
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Note that, for the present (small) molecular systems, {\GOWO}@HF and ev$GW$@HF yield similar quasiparticle energies and fundamental gap.
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Moreover, {\GOWO} allows to avoid rather laborious iterations as well as the significant additional computational effort of ev$GW$.
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As one-electron basis sets, we employ the Dunning families (cc-pVXZ and aug-cc-pVXZ) defined with cartesian Gaussian functions.
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In the present study, the zeroth-order Hamiltonian [see Eq.~\eqref{eq:LR-PT}] is always the ``full'' BSE static Hamiltonian, \ie, without TDA.
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The dynamical correction, however, is computed in the dTDA throughout.
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As one-electron basis sets, we employ the Dunning families cc-pVXZ and aug-cc-pVXZ (X = D, T, and Q) defined with cartesian Gaussian functions.
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Finally, the infinitesimal $\eta$ is set to $100$ meV for all calculations.
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For comparison purposes, we employ the theoretical best estimates (TBEs) and geometries of Refs.~\onlinecite{Loos_2018a,Loos_2019,Loos_2020b} from which CIS(D), \cite{Head-Gordon_1994,Head-Gordon_1995} ADC(2), \cite{Trofimov_1997,Dreuw_2015} CC2, \cite{Christiansen_1995a} CCSD, \cite{Purvis_1982} and CC3 \cite{Christiansen_1995b} excitation energies are also extracted.
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@ -760,7 +762,6 @@ All the static and dynamic BSE calculations have been performed with the softwar
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\begin{table*}
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\caption{
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Singlet and triplet excitation energies (in eV) of \ce{N2} computed at the BSE@{\GOWO}@HF level for various basis sets.
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The dynamical correction is computed in the dTDA.
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\label{tab:N2}
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}
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\begin{ruledtabular}
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@ -814,24 +815,21 @@ All the static and dynamic BSE calculations have been performed with the softwar
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\end{squeezetable}
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First, we investigate the basis set dependency of the dynamical correction.
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Note that, in the present calculations, the zeroth-order Hamiltonian is always the ``full'' BSE static Hamiltonian, \ie, without TDA.
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The singlet and triplet excitation energies of the nitrogen molecule \ce{N2} computed at the BSE@{\GOWO}@HF level for the aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets are reported in Table \ref{tab:N2}, where we also report the $GW$ gap, $\Eg^{\GW}$, to show that corrected transitions are usually well below this gap.
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The \ce{N2} molecule is a very convenient example as it contains $n \ra \pis$ and $\pi \ra \pis$ valence excitations as well as Rydberg transitions.
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The singlet and triplet excitation energies of the nitrogen molecule \ce{N2} computed at the BSE@{\GOWO}@HF level for the cc-pVXZ and aug-cc-pVXZ families of basis sets are reported in Table \ref{tab:N2}, where we also report the $GW$ gap, $\Eg^{\GW}$, to show that corrected transitions are usually well below this gap.
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The \ce{N2} molecule is a very convenient example for this kind of study as it contains $n \ra \pis$ and $\pi \ra \pis$ valence excitations as well as Rydberg transitions.
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As we shall further illustrate below, the magnitude of the dynamical correction is characteristic of the type of transitions.
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One key result of the present investigation is that the dynamical correction is quite basis set insensitive with a maximum variation of $0.03$ eV between in smallest (aug-cc-pVDZ) and largest (aug-cc-pVQZ) basis sets.
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This is quite a nice feature as one does not need to compute this more expensive correction in a very large basis.
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This is quite a nice feature as it means that one does not need to compute the dynamical correction in a very large basis to get a meaningful estimate of its magnitude.
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%The second important observation extracted from the results gathered in Table \ref{tab:N2} is that the dTDA is a rather satisfactory approximation, especially for the singlet states where one observes a maximum discrepancy of $0.03$ eV between the ``full'' and dTDA excitation energies.
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%The story is different for the triplet states for which deviations of the order of $0.3$ eV is observed between the two sets, the dTDA of excitation energies being, as we shall see later on, more accurate.
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%Indeed, although the dynamical correction systematically red-shift the excitation energies (as anticipated in Sec.~\ref{sec:dynW}), taking into account the coupling between the resonant and anti-resonant parts of the BSE Hamiltonian [see Eq.~\eqref{eq:BSE-final}] yields a systematic blue-shift of the correction, the overall correction remaining negative but by a smaller amount.
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%This outcome is similar to the conclusions of several benchmark studies \cite{Jacquemin_2017b,Rangel_2017} which clearly concluded that static BSE triplet excitations are notably too low in energy and that the use of the TDA is able to partly reduce this error.
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%In accordance with the success of the dTDA, the remaining calculations of the present study are performed within this approximation.
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%%% TABLE I %%%
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\begin{squeezetable}
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\begin{table*}
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\caption{
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Singlet excitation energies (in eV) for various molecules obtained with the aug-cc-pVTZ basis set computed at various levels of theory.
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The dynamical correction is computed in the dTDA.
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CT stands for charge transfer.
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\label{tab:BigTabSi}
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}
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@ -916,6 +914,12 @@ This is quite a nice feature as one does not need to compute this more expensive
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& 0.41 & 0.24 & 0.14 & 0.25 & 0.00 \\
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MSE & & & & 0.65 & 0.48 & &
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& 0.12 & 0.00 & 0.13 & 0.00 & 0.00 \\
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RMSE & & & & 0.71 & 0.58 & &
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& 0.54 & 0.34 & 0.19 & 0.33 & 0.00 \\
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Max(+) & & & & 1.08 & 0.91 & &
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& 1.06 & 0.54 & 0.44 & 0.57 & 0.00 \\
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Max(-) & & & & 0.20 & -0.22 & &
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& -1.77 & -0.76 & -0.02 & -0.71 & 0.00 \\
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\end{tabular}
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\end{ruledtabular}
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\end{table*}
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@ -926,7 +930,6 @@ This is quite a nice feature as one does not need to compute this more expensive
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\begin{table*}
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\caption{
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Triplet excitation energies (in eV) for various molecules obtained with the aug-cc-pVTZ basis set computed at various levels of theory.
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The dynamical correction is computed in the dTDA.
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\label{tab:BigTabTr}
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}
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\begin{ruledtabular}
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@ -998,6 +1001,25 @@ This is quite a nice feature as one does not need to compute this more expensive
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\end{table*}
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\end{squeezetable}
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%%% FIG I %%%
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\begin{figure*}
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\includegraphics[width=\linewidth]{fig1a}
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\includegraphics[width=\linewidth]{fig1b}
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\caption{Error (in eV) with respect to the TBEs of Refs.~\onlinecite{Loos_2018a,Loos_2019,Loos_2020b} for singlet (top) and triplet (bottom) excitation energies of various molecules obtained with the aug-cc-pVTZ basis set computed within the static (white) and dynamic (colored) BSE formalism.
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CT and R stand for charge transfer and Rydberg state, respectively.
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See Tables \ref{tab:BigTabSi} and \ref{tab:BigTabTr} for raw data.
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\label{fig:SiTr-SmallMol}}
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\end{figure*}
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%%% FIG II %%%
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\begin{figure*}
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\includegraphics[width=\linewidth]{fig2}
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\caption{Error (in eV) with respect to CC3 for singlet and triplet excitation energies of various molecules obtained with the aug-cc-pVDZ basis set computed within the static (white) and dynamic (colored) BSE formalism.
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R stands for Rydberg state.
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See Table \ref{tab:BigMol} for raw data.
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\label{fig:SiTr-BigMol}}
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\end{figure*}
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Tables \ref{tab:BigTabSi} and \ref{tab:BigTabTr} report, respectively, singlet and triplet excitation energies for various molecules computed at the BSE@{\GOWO}@HF level and with the aug-cc-pVTZ basis set.
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For comparative purposes, excitation energies obtained with the same basis set and several second-order wave function methods [CIS(D), ADC(2), CCSD, and CC2] are also reported.
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Finally, the highly-accurate TBEs of Refs.~\onlinecite{Loos_2018a,Loos_2019,Loos_2020b} will serve us as reference.
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@ -1005,12 +1027,11 @@ For each excitation, we report the static and dynamic excitation energies, $\Om{
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As one can see in Tables \ref{tab:BigTabSi} and \ref{tab:BigTabTr}, the value of $Z_S$ is always quite close to unity which shows that the perturbative expansion behaves nicely, and that a first-order correction is probably quite a good estimate of the non-perturbative result.
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Moreover, we have observed that an iterative, self-consistent resolution [where the dynamically-corrected excitation energies are re-injected in Eq.~\eqref{eq:Om1}] yields basically the same results as its (cheaper) renormalized version.
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%%% TABLE I %%%
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%%% TABLE III %%%
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\begin{squeezetable}
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\begin{table}
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\caption{
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Singlet and triplet excitation energies (in eV) for various molecules obtained with the aug-cc-pVDZ basis set computed at various levels of theory.
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The dynamical correction is computed in the dTDA.
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\label{tab:BigMol}
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}
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\begin{ruledtabular}
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