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% \def\E{{\cal E}}
% \def\EE{{\displaystyle{\pmb{\cal E}}}}
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\begin{document}
\begin{center}
{\large\bf
Notes for the implementation of constant electric field\\
and constant electric displacement field\\
methods in \ABINIT }
\par\bigskip
D. Vanderbilt and J. Hong\\
May 21, 2012
\end{center}
\vskip 0.4cm
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Introduction}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
The purpose of these notes is to provide the theoretical
and formal background for the implementation of the constant
electric field~\cite{siv} and constant electric displacement
field~\cite{ssv} methods in the context of first-principles
electronic structure theory, in particular as they are
implementated in the \ABINIT~\cite{abinit} code package. The
full theory can be found in the above two references. Most of
the present notes are adopted from the supplementary material
of Ref.~\cite{ssv}.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Energy functionals}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% :::::::::::::::::::::::::::::::::::::::::::
\subsection{Units}
% :::::::::::::::::::::::::::::::::::::::::::
We use Gaussian units so that $\D=\EE+4\pi\P$, etc., where we use
$\EE$ to denote the electric field. Energies like
$E$, $U$, $\F$ and $\Ft$ are {\em energies per unit cell} with
units of energy.
% :::::::::::::::::::::::::::::::::::::::::::
\subsection{Energy functionals for constant field calculation}
% :::::::::::::::::::::::::::::::::::::::::::
Internal energy $U$ is introduced for constant displacement field
calculation~\cite{ssv}:
\begin{equation}
U(\mathbf{D},v) = E_{\rm KS}(v) +
\frac{\Omega}{8\pi} [ \mathbf{D} - 4\pi \mathbf{P}(v) ]^2 \;.
\label{fixd}
\end{equation}
$U(\mathbf{D},v)$ depends directly on an external vector parameter
$\mathbf{D}$, and indirectly on the internal (ionic and electronic)
coordinates $v$ through the Kohn-Sham energy $E_{\rm KS}$ and the
Berry-phase polarization $\mathbf{P}$~\cite{kv}.
The electric enthalpy $\F$ is introduced for constant $\E$
calculation:~\cite{siv}
%
\begin{equation}
\mathcal{F}(\bm{\mathcal{E}},v) = E_{\rm KS}(v) - \Omega\, \bm{\mathcal{E}} \cdot \mathbf{P}(v)
\;.
\label{fixe}
\end{equation}
%
However, according to Ref.~\cite{ssv}, $\Ft$ is a more natural
functional to be used in constant $\E$ calculation:
\begin{equation}
\tilde{\mathcal{F}} = U - \frac{\Omega}{4\pi} \EE \cdot \mathbf{D}
= \mathcal{F} - \frac{\Omega}{8\pi} \mathcal{E}^2 \;.
\label{fixet}
\end{equation}
%
(Since $\mathcal{F}$ and $\tilde{\mathcal{F}}$ only differ by a function
of $\EE$, both of them yield the same equilibrium state at fixed $\EE$.)
% :::::::::::::::::::::::::::::::::::::::::::
\section{Strains and strain derivatives}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%==========================================
\subsection{Introducing reduced field variables}
%==========================================
For treating variable strain, it is strongly advantageous to change
to internal variables.
To define internal variables for the fields, we let $\a_j$ be the
lattice vectors, and $g_{ij}=\a_i\cdot \a_j$ be the metric.
We also let $\b_i$ be dual vectors defined as $\mathbf{a}_i\cdot\mathbf{b}_j=\delta_{ij}$,
in which the conventional factor of $2\pi$ is {\it not included}, so that
$\b_i\cdot\b_j=(g^{-1})_{ij}$.
There are now two choices of reduced variables. Referencing to
the reciprocal vectors, we get reduced variables
%
\beq
p_i=\Omega\,\b_i\cdot\P
\qquad \Longleftrightarrow \qquad
\P=\frac{1}{\O}\,\sum_i p_i\,\a_i \;,
\eeq
\beq
\ee_i=\frac{\Omega}{4\pi}\,\b_i\cdot\EE
\qquad \Longleftrightarrow \qquad
\EE=\frac{4\pi}{\O}\,\sum_i \ee_i\,\a_i \;,
\eeq
\beq
d_i = \frac{\Omega}{4\pi}\,\b_i\cdot\D
\qquad \Longleftrightarrow \qquad
\D=\frac{4\pi}{\O}\,\sum_i d_i\,\a_i \;.
\eeq
%
where the inverse relations are given to the right.
The relation $\D=\EE+4\pi\P$ becomes
%
\beq
d_i=\ee_i+p_i.
\eeq
%
The reduced variables $d_i$, $\ee_i$, and $p_i$ have units of charge,
and are related to the free charge, total charge, and bound charge,
respectively, found on a surface of orientation $\hat{\b}_i$ if the
fields vanish in the vacuum.
Note that, aside from a factor of $e/2\pi$, the $p_i$ are nothing other
than the Berry phases $\phi_i$ as given, e.g., in Eq.~(23) of
Ref.~\cite{proper}.
The other choice is to refer to the real-space lattice
vectors, i.e.,
%
\beq
\pb_i= 4\pi\,\a_i\cdot\P
\qquad \Longleftrightarrow \qquad
\P= \frac{1}{4\pi}\, \sum_i \pb_i\,\b_i \;,
\eeq
\beq
\eb_i= \a_i\cdot\EE
\qquad \Longleftrightarrow \qquad
\EE= \sum_i \eb_i\,\b_i \;,
\eeq
\beq
\db_i= \a_i\cdot\D
\qquad \Longleftrightarrow \qquad
\D= \sum_i \db_i\,\b_i \;.
\eeq
%
The relation $\D=\EE+4\pi\P$ becomes
%
\beq
\db_i=\eb_i+\pb_i.
\eeq
%
The reduced variables $\pb_i$, $\eb_i$, and $\db_i$ have units of
electric potential (energy/charge),
and are related to the potential drop across the unit cell in
direction $\hat{\a}_i$ arising from the displacement field, the
total field, and the depolarization field, respectively. They
are related to the unbarred quantities by
%
\beq
\pb_i=\foo\,\gg\,p_j \;,
\qquad
\eb_i=\foo\,\gg\,\ee_j \;,
\qquad
\db_i=\foo\,\gg\,d_j \;,
\eeq
where an implied sum notation is used.
The reduced field variables introduced here are closely related to
those discussed in Ref.~\cite{siv} (see, e.g., Eq.~(5) therein) and
in Sec.~II.C.3 and the Appendix of of Ref.~\cite{wvh}.
Eqs.~(A4) and (A5) of Ref.~\cite{wvh} introduce field
variables that are reminiscent of $p_i$ and $\eb_i$ here,
but there they were defined in such a way as to coincide
with the ordinary $\P$ and $\EE$ in the absence of strains or
rotations. More closely related are the $\P'$ and $\ee_\mu$
variables defined in (A13) and (A14) of Ref.~\cite{wvh}, which are identical
to our $p_i$ and $\eb_i$ except for a factor of the charge quantum
$e$.
%=======================================
\subsection{Energy functionals in reduced variables}
%=======================================
The equation analogous to Eq.~(\ref{fixe}) is
%
\beq
\F(\eb)=E(p)-\foo\,\gg\,\ee_i\,p_j
=E(p)-\eb_i\,p_i \;.
\label{eq:fixeb}
\eeq
%
Note that the natural variable of function $\F$ is $\eb$, not $\ee$.
That is, the variable conjugate to $p_i$ is
$(4\pi/\O)\gg\ee_j=\eb_i$. This is the reason why we recommend to
use $\eb$ in the constant electric field calculation (as implemented
in \ABINIT).
We also have
%
\beq
\eb_i=\frac{dE}{dp_i} \;,\qquad p_i=-\,\frac{d\F}{d\eb}\;.
\eeq
%
Then Eq.~(\ref{fixet}) becomes\footnote{The volume $\O$ was erroneously
omitted in Eq.(33) of the supplementary notes of Ref.~\cite{ssv}.}
%
\beq
\Ft(\eb)=\F(\eb)-\frac{\O}{8\pi}\,(g^{-1})_{ij}\,\eb_i\,\eb_j
=E(p)-\eb_i\,p_i-\frac{\O}{8\pi}\,(g^{-1})_{ij}\,\eb_i\,\eb_j
\eeq
%
and Eq.~(\ref{fixd}) becomes
%
\beq
U=E+\too\,\gg\,\ee_i\,\ee_i
=E+\frac{1}{2}\,\eb_i\,\ee_i
=E+\frac{\O}{8\pi}\,(g^{-1})_{ij}\,\eb_i\,\eb_j\;.
\label{eq:fixdred}
\eeq
We also have
%
\beq
d_i=-\,\frac{d\Ft}{d\eb_i} \;,\qquad \eb_i=\frac{dU}{dd_i} \;.
\eeq
For the electric enthalpy function, we can imagine a large number
$N$ of crystalline cell layers sandwiched between capacitor
electrodes with a voltage $V$ applied across the electrodes. If
the cell is strained as a result of the applied voltage or for any
other reason, the voltage drop per cell will remain $V/N$,
corresponding to a fixed $\eb$. It thus makes sense that this is
the natural variable for this kind of problem. On the other hand,
the variable $\ee$ would change with strain, and so is not an
appropriate choice of variable in this context.
On the other hand, instead of a capacitor with fixed voltage
across the plates, we can imagine a slab with fixed free charge
on the surfaces. More precisely, it would be fixed free
charge per surface cell, not per unit area, under general strain
deformations. This corresponds to fixed $d$, and so it is
natural that $U(d)$ has natural variable $d$, not $\db$.
%=======================================
\subsection{Strain, strain derivatives, and the stress tensor}
%=======================================
Let $\emn$ be the strain tensor, and define the stresses
$\sigma_{\mu\nu}^E=\Omega^{-1}dE/d\emn$,
$\sigma_{\mu\nu}^\F=\Omega^{-1}d\F/d\emn$,
$\sigma_{\mu\nu}^{\Ft}=\Omega^{-1}d\Ft/d\emn$, and
$\sigma_{\mu\nu}^U=\Omega^{-1}dU/d\emn$.
Then
%
\beq
\frac{d\O}{d\emn}=\O\delta_{\mu\nu}
\label{AA}
\eeq
%
and
%
\beq
\frac{d\gg}{d\emn} = a_{i\mu}\,a_{j\nu} + a_{j\mu}\,a_{i\nu} \;.
\label{BB}
\eeq
%
The Hellmann-Feynman theorem applied to the electric enthalphy is
%
\beq
\left(\frac{d\F(\eb,\eta;v)}{d\emn}\right)_{\eb}=
\frac{\partial \F(\eb,\eta;v)}{\partial\emn} +
\frac{\partial \F(\eb,\eta;v)}{\partial v}\,\frac{dv}{d\emn}
\eeq
%
but since $\partial \F/\partial v=0$ at the equilibrium state of the internal
variables $\{v\}$, the second term vanishes. Using
$\F(\eb) =E(p)-\eb_i\,p_i$ we find
%
\beq
\frac{d\F(\eb,\eta)}{d\emn}=\frac{\partial E(p,\eta)}{\partial\emn}
-\eb_i\,\frac{\partial p_i}{\partial\emn} \;.
\eeq
%
But if we assume that the internal variables are atomic
coordinates in lattice-vector units and coefficients of plane-wave
basis functions in a norm-conserving context, it follows that
$\partial p_i/\partial\emn=0$. Thus
%
\beq
\sigma^\F_{\mu\nu}=
\frac{1}{\O}\,\frac{d\F}{d\emn}=
\frac{1}{\O}\,\frac{\partial E}{\partial\emn} = \sigma^E_{\mu\nu}
\eeq
%
which is just the stress tensor appearing in the usual KS theory.
In the case of USPP or PAW approaches, $\partial p_i/\partial\emn$
does not vanish, and augmentation terms need to be included.
For the internal energy, we again use the Hellmann-Feynman argument
to write $dU/d\emn=\partial U/\partial d\emn$. Now the natural
variable being held fixed is $d$, and again $p$ is unchanged under
a homogeneous strain if the internal variables are chosen properly,
and since $d_i=\ee_i+p_i$, this means $\ee$ is also fixed (while
$\eb$ is not). We choose to write Eq.~(\ref{eq:fixdred}) as
%
\beq
U(\eta,d)=E+\too\,\gg\,\ee_i\,\ee_j
\eeq
%
so that, using Eqs.~(\ref{AA}) and (\ref{BB}),
%
\beq
\sigma^U_{\mu\nu}=
\frac{1}{\O}\,\frac{dU}{d\emn}
= \frac{1}{\O}\,\frac{\partial E}{\partial\emn}
+\,\frac{2\pi}{\O^2}\,
\Big[\,
2\,a_{i\mu}\,a_{j\nu}\,\ee_i\,\ee_j -\delta_{\mu\nu}\,\gg\,\ee_i\,\ee_i
\,\Big]
\eeq
%
or
%
\beq
\sigma^U_{\mu\nu}=\sigma^{\rm KS}_{\mu\nu}+\,\frac{1}{8\pi}\,
\Big[\, 2\,\E_\mu\,\E_\nu - \delta_{\mu\nu}\,\E^2 \,\Big]
\eeq
%
where the second term is just the Maxwell stress tensor arising from the
macroscopic electric field. It is straightforward to show that
$\sigma^{\Ft}=\sigma^U$. Thus, there are basically two stress
tensors, one ($\sigma^\F_{\mu\nu}=\sigma^E_{\mu\nu}$) that does not include
the Maxwell stress, and another ($\sigma^{\Ft}=\sigma^U$) that does.
%=======================================
\section{\ABINIT\ implementation}
%=======================================
In the \ABINIT\ implementation, $\Ft$ and $U$ have been chosen as
the energy functionals for fixed electric field or fixed electric
displacement field cases, respectively. The corresponding
fields are the reduced variables $\eb_i$ or $d_i$, in
units of a.u. Therefore, the stress tensor includes the Maxwell
stress. The ion positions and cell parameters can be optimized
either under fixed $\eb_i$ or fixed $d_i$ boundary conditions.
This mode of operation, in which the user inputs target reduced
fields for the constrained calculation, is the recommended one
when the lattice vectors are going to be relaxed.
When the cell parameters are \textit{not} going to be relaxed, it
may be more convenient to work with the unreduced field variables
$\EE$ or $\D$ instead; \ABINIT\ also allows this option, in which
the unreduced target field is input directly (in a.u.).
The \ABINIT\ implementation does also allow calculations at fixed
unreduced $\EE$ or $\D$ with relaxation of the cell parameters,
but this option should be used with caution. It is important to
note that the energy functionals of Eq.~(\ref{eq:fixeb})
and Eq.~(\ref{eq:fixdred}) are still used in these cases.
Thus, when working at fixed $\EE$ for example, the code
searches for a value of $\eb_i$ such that the equilibrium
structure at fixed $\eb_i$ has the unreduced field $\EE$ matching
the target one. This procedure is not variational in the usual
sense. During such a run, the reported forces and
stresses that are used to guide the minimization are not, in
principle, equal to the numerical derivatives of the energy
functional. For these reasons, it is recommended to choose
the reduced-field options when relaxing the cell parameters
along with the internal coordinates.
Relaxed-cell calculations at fixed $\E$ and $D$ along
one dimension were implemented first by M.~Stengel in a private
code package ({\tt LAUTREC}) as described in Ref.~\cite{ssv}
and references therein. Tests of the current \ABINIT\
implementation (which supports fixed three-dimensional $\EE$ or
$\D$) were presented in Ref.~\cite{hv}, showing excellent agreement
with the {\tt LAUTREC} implementation.
%===============================
%===============================
\begin{thebibliography}{99}
%===============================
%===============================
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\bibitem{ssv}
Massimiliano Stengel, Nicola A. Spaldin, and David Vanderbilt,
Nat Phys {\bf 5}, 304 (2009).
\bibitem{abinit} {\sf ABINIT} is a common project of the
Universit\'{e} Catholique de Louvain, Corning Incorporated, and
other contributors (https://www.abinit.org). X. Gonze, J.-M.
Beuken, R. Caracas, F. Detraux, M. Fuchs, G.-M. Rignanese, L.
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\bibitem{kv}
R.~D. King-Smith, David Vanderbilt, Phys. Rev. B, {\bf 47},R1651,(1993).
\bibitem{proper}
D. Vanderbilt, J. Phys. Chem. Solids {\bf 61}, 147 (2000).
\bibitem{wvh}
X. Wu, D. Vanderbilt, and D.R. Hamann, Phys. Rev. B {\bf 72}, 035105 (2005).
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%\bibitem{dieguez}
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\end{thebibliography}
\end {document}