Modified the theory
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@ -321,25 +321,21 @@ We refer the interested readers to Refs.~\onlinecite{GinPraFerAssSavTou-JCP-18,L
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The local-density approximation (LDA) of the ECMD complementary functional is defined as
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\begin{equation}
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\label{eq:def_lda_tot}
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\bE{\LDA}{}[\n{}{},\rsmu{}{}] = \int \n{}{}(\br{}) \be{\text{c,md}}{\sr,\LDA}\qty(\n{}{}(\br{}),\zeta(\br{}),\rsmu{}{}(\br{})) \dbr{},
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\bE{\LDA}{\Bas}[\n{}{},\rsmu{}{\Bas}] = \int \n{}{}(\br{}) \be{\text{c,md}}{\sr,\LDA}\qty(\n{}{}(\br{}),\zeta(\br{}),\rsmu{}{\Bas}(\br{})) \dbr{},
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\end{equation}
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where $\zeta = (\n{\uparrow}{} - \n{\downarrow}{})/\n{}{}$ is the spin polarization and $\be{\text{c,md}}{\sr,\LDA}(\n{}{},\zeta,\rsmu{}{})$ is the ECMD short-range correlation energy per electron of the uniform electron gas (UEG) \cite{LooGil-WIRES-16} parameterized in Ref.~\citenum{PazMorGorBac-PRB-06}.
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The ECMD LDA functional \eqref{eq:def_lda_tot} presents two main defects: i) at small $\mu$, it overestimates the correlation energy, and ii) UEG-based quantities are hardly transferable when the system becomes strongly correlated or multi-configurational.
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The $\be{\text{c,md}}{\sr,\LDA}$ used in \eqref{eq:def_lda_tot} presents two main defects: i) at small $\mu$, it overestimates the correlation energy, and ii) UEG-based quantities are hardly transferable when the system becomes strongly correlated or multi-configurational.
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An attempt to solve these problems has been proposed by some of the authors in the context of RS-DFT. \cite{FerGinTou-JCP-18}
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They proposed to interpolate between the exact large-$\mu$ behavior \cite{TouColSav-PRA-04, GorSav-PRA-06, PazMorGorBac-PRB-06} and the Perdew-Burke-Ernzerhof (PBE) functional \cite{PerBurErn-PRL-96} $\e{\text{c}}{\PBE}(\n{}{},s,\zeta)$ at $\mu = 0$.
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The PBE correlation functional has clearly shown to improve energetics for small $\mu$, \cite{LooPraSceTouGin-JPCL-19} and the exact behavior at large $\mu$ naturally introduces the \textit{exact} on-top pair density $\n{2}{}(\br{},\br{}) \equiv \n{2}{}(\br{})$ which contains information about the level of strong correlation.
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\titou{Obviously, $\n{2}{}(\br{})$ cannot be accessed in practice} and must be approximated by a function referred here as $\tn{2}{}(\br{})$.
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Therefore, based on the proposition of Ref.~\onlinecite{FerGinTou-JCP-18}, we introduce the general form of the PBE complementary functional:
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\begin{multline}
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\label{eq:def_pbe_tot}
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\bE{\PBE}{\manu{\Bas}}[\n{}{},\tn{2}{},\rsmu{}{\manu{\Bas}}] =
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\int \n{}{}(\br{})
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\\
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\times \be{\text{c,md}}{\sr,\PBE}\qty(\n{}{}(\br{}),\tn{2}{}(\br{}),s(\br{}),\zeta(\br{}),\rsmu{}{\manu{\Bas}}(\br{})) \dbr{},
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\end{multline}
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with
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They proposed to interpolate between the usual Perdew-Burke-Ernzerhof (PBE) correlation functional \cite{PerBurErn-PRL-96} $\e{\text{c}}{\PBE}(\n{}{},s,\zeta)$ at $\mu = 0$ and the exact large-$\mu$ behavior known from RS-DFT\cite{TouColSav-PRA-04, GorSav-PRA-06, PazMorGorBac-PRB-06}.
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\manu{In the context of RS-DFT, the large-$\mu$ behavior corresponds to an extremely short-range interaction, and in that regime the ECMD energy only depends on the \textit{exact} on-top pair density $\n{2}{}(\br{},\br{})$
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\begin{align}
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\label{eq:exact_large_mu}
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\bE{\text{c,md}}{\sr}[\n{2}{},\rsmu{}{}] \propto \frac{1}{\mu^3} \int \dbr{} \n{2}{}(\br{},\br{}) + O(\frac{1}{\mu^4}),
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\end{align}
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where $\n{2}{}(\br{},\br{})$ is obtained from the exact ground state wave function $\Psi$ developed in an complete basis set.
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Obviously, an exact quantity such as $\n{2}{}(\br{},\br{})$ is out of reach in practical calculations and must be approximated by a function generally referred here as $\tn{2}{}(\br{},\br{})$.
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For a given choice of $\tn{2}{}(\br{},\br{})$, the authors in Ref~\cite{FerGinTou-JCP-18} proposed to use the following functional form for the correlation energy density in order to interpolate between the PBE at $\mu = 0$ and the equation \eqref{eq:exact_large_mu}:
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\begin{subequations}
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\begin{gather}
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\label{eq:epsilon_cmdpbe}
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@ -349,8 +345,18 @@ with
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\beta^\PBE(\n{}{},\tn{2}{},s,\zeta) = \frac{3}{2\sqrt{\pi} (1 - \sqrt{2})} \frac{\e{\text{c}}{\PBE}(\n{}{},s,\zeta)}{\tn{2}{}/\n{}{}}.
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\end{gather}
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\end{subequations}
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As illustrated in the context of RS-DFT in Ref~\cite{FerGinTou-JCP-18}, such a functional form is able to treat both the weak correlation regime (thanks to the use of the $\e{\text{c}}{\PBE}$) and the strong correlation regime (thanks to good approximations of the on-top pair density $\tn{2}{}$).\\
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Therefore, in the context of the basis set correction we use the explicit form of equations ~\eqref{eq:epsilon_cmdpbe}~and~\eqref{eq:beta_cmdpbe} with $\rsmu{}{\manu{\Bas}}$ and introduce the general form of the PBE-based complementary functional: }
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\begin{multline}
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\label{eq:def_pbe_tot}
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\bE{\PBE}{\manu{\Bas}}[\n{}{},\tn{2}{},\rsmu{}{\manu{\Bas}}] =
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\int \n{}{}(\br{})
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\\
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\times \be{\text{c,md}}{\sr,\PBE}\qty(\n{}{}(\br{}),\tn{2}{}(\br{}),s(\br{}),\zeta(\br{}),\rsmu{}{\manu{\Bas}}(\br{})) \dbr{},
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\end{multline}
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\manu{which depends on the approximation used for $\tn{2}{}$.}
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In Ref.~\onlinecite{LooPraSceTouGin-JPCL-19}, some of the authors introduced a new PBE-based functional, here-referred as \titou{PBE-UEG},
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\manu{In Ref.~\onlinecite{LooPraSceTouGin-JPCL-19}, some of the authors introduced a version of the PBE-based functional, here-referred as \titou{PBE-UEG}:}
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\begin{multline}
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\label{eq:def_pbe_tot}
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\bE{\titou{\PBE\text{-}\UEG}}{\manu{\Bas}}[\n{}{},\n{2}{\UEG},\rsmu{}{\manu{\Bas}}] =
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@ -358,15 +364,22 @@ In Ref.~\onlinecite{LooPraSceTouGin-JPCL-19}, some of the authors introduced a n
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\\
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\times \be{\text{c,md}}{\sr,\PBE}\qty(\n{}{}(\br{}),\n{2}{\UEG}(\br{}),s(\br{}),\zeta(\br{}),\rsmu{}{\manu{\Bas}}(\br{})) \dbr{},
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\end{multline}
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in which the on-top pair density was approximated by its UEG version,
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in which the on-top pair density was approximated by its UEG version, \manu{\textit{i.e.} $\tn{2}{}(\br{}) = \n{2}{\UEG}(\br{})$, with}
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\begin{equation}
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\n{2}{\UEG}(\br{}) = n(\br{})^2 (1-\zeta(\br{})^2) g_0(n(\br{})),
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\end{equation}
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where $g_0(n)$ is the UEG on-top pair distribution function [see Eq.~(46) of Ref.~\citenum{GorSav-PRA-06}].
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and where $g_0(n)$ is the UEG on-top pair distribution function [see Eq.~(46) of Ref.~\citenum{GorSav-PRA-06}].
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\manu{As illustrated in Ref~\cite{LooPraSceTouGin-JPCL-19} for weakly correlated systems, this PBE-based functional has clearly shown to improve energetics over the pure UEG-based functional $\bE{\LDA}{\Bas}$ (see \eqref{eq:def_lda_tot}) thanks to the inclusion of the PBE functional.}
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As mentioned earlier, the incorporation of the PBE functional as a limiting form at $\mu = 0$ [see Eqs.~\eqref{eq:epsilon_cmdpbe} and \eqref{eq:beta_cmdpbe}] has shown to significantly improve the energetic properties over the LDA for weakly correlated systems.
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However, the underlying UEG on-top pair density might not be suited for the treatment of excited states and/or strongly correlated systems.
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Therefore, we propose here the "PBE-ontop" (PBEot) functional,
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\manu{Also, in the context of the basis set correction, we have to compute the on-top pair density $\n{2}{\manu{\Bas}}(\br{})$ to compute $\rsmu{}{\manu{\Bas}}(\br{})$ (see equations~\eqref{eq:def_mu} and \eqref{eq:def_weebasis}), and therefore we can use $\n{2}{\manu{\Bas}}(\br{})$ to have an approximation of the exact on-top pair density.
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More precisely, we propose an approximation of the on-top pair density $\ttn{2}{\manu{\Bas}}(\br{})$ defined as
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\begin{equation}
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\ttn{2}{\manu{\Bas}}(\br{}) = \n{2}{\manu{\Bas}}(\br{}) \qty( 1 + \frac{2}{\sqrt{\pi}\rsmu{}{\manu{\Bas}}(\br{})})^{-1}
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\end{equation}
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which is inspired by the extrapolation at large $\mu$ of the exact on-top pair density proposed by Gori-Giorgi and Savin\cite{GorSav-PRA-06} in the context of RS-DFT.
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}
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Therefore, we propose here the "PBE-ontop" (PBEot) functional which uses the on-top pair density computed in the basis set $\Bas$ as ingredient:
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\begin{multline}
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\label{eq:def_pbe_tot}
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\bE{\PBEot}{\manu{\Bas}}[\n{}{},\ttn{2}{\manu{\Bas}},\rsmu{}{\manu{\Bas}}] =
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@ -374,10 +387,6 @@ Therefore, we propose here the "PBE-ontop" (PBEot) functional,
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\\
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\times \be{\text{c,md}}{\sr,\PBE}\qty(\n{}{}(\br{}),\ttn{2}{\manu{\Bas}}(\br{}),s(\br{}),\zeta(\br{}),\rsmu{}{\manu{\Bas}}(\br{})) \dbr{},
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\end{multline}
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a variant inspired by the work of Ref.~\onlinecite{FerGinTou-JCP-18} where the exact on-top pair density is approximated by the extrapolation formula proposed by Gori-Giorgi and Savin:\cite{GorSav-PRA-06}\manu{. In the present context, we use the on-top pair density in the basis set $\Bas$ $ \n{2}{\manu{\Bas}}(\br{})$ together with the associated range separation function $\rsmu{}{\Bas}(\br{})$, which leads to the following approximated on-top pair density: }
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\begin{equation}
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\ttn{2}{\manu{\Bas}}(\br{}) = \n{2}{\manu{\Bas}}(\br{}) \qty( 1 + \frac{2}{\sqrt{\pi}\rsmu{}{\manu{\Bas}}(\br{})})^{-1}.
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\end{equation}
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%Such formula relies on the association between $\n{2}{\Bas}(\br{ },\br{ })$, which is the on-top pair density computed in the basis set $\Bas$ in a given point, and the local value of the range-separation parameter $\rsmu{}{\Bas}$ at the same point.
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The sole distinction between \titou{PBE-UEG} and PBEot is the level of approximation of the exact on-top pair density.
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