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\pdf_author "Marek Nečada"
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\begin_body

\begin_layout Section
Infinite periodic systems
\begin_inset CommandInset label
LatexCommand label
name "sec:Infinite"

\end_inset


\begin_inset FormulaMacro
\newcommand{\dlv}{\vect a}
\end_inset


\begin_inset FormulaMacro
\newcommand{\rlv}{\vect b}
\end_inset


\end_layout

\begin_layout Standard
Although large finite systems are where MSTMM excels the most, there are
 several reasons that makes its extension to infinite lattices (where periodic
 boundary conditions might be applied) desirable as well.
 Other methods might be already fast enough, but MSTMM will be faster in
 most cases in which there is enough spacing between the neighboring particles.
 MSTMM works well with any space group symmetry the system might have (as
 opposed to, for example, FDTD with cubic mesh applied to a honeycomb lattice),
 which makes e.g.
 application of group theory in mode analysis quite easy.
\begin_inset Note Note
status open

\begin_layout Plain Layout
Topology anoyne?
\end_layout

\end_inset

 And finally, having a method that handles well both infinite and large
 finite system gives a possibility to study finite-size effects in periodic
 scatterer arrays.
\end_layout

\begin_layout Subsection
Formulation of the problem
\end_layout

\begin_layout Standard
Let us have a linear system of compact EM scatterers on a homogeneous background
 as in Section 
\begin_inset CommandInset ref
LatexCommand eqref
reference "subsec:Multiple-scattering"
plural "false"
caps "false"
noprefix "false"

\end_inset

, but this time, the system shall be periodic: let there be a 
\begin_inset Formula $d$
\end_inset

-dimensional (
\begin_inset Formula $d$
\end_inset

 can be 1, 2 or 3) Bravais lattice embedded into the three-dimensional real
 space, with lattice vectors 
\begin_inset Formula $\left\{ \dlv_{i}\right\} _{i=1}^{d}$
\end_inset

, and let the lattice points be labeled with an 
\begin_inset Formula $d$
\end_inset

-dimensional integar multiindex 
\begin_inset Formula $\vect n\in\ints^{d}$
\end_inset

, so the lattice points have cartesian coordinates 
\begin_inset Formula $\vect R_{\vect n}=\sum_{i=1}^{d}n_{i}\vect a_{i}$
\end_inset

.
 There can be several scatterers per unit cell with indices 
\begin_inset Formula $\alpha$
\end_inset

 from set 
\begin_inset Formula $\mathcal{P}_{1}$
\end_inset

 and (relative) positions inside the unit cell 
\begin_inset Formula $\vect r_{\alpha}$
\end_inset

; any particle of the periodic system can thus be labeled by a multiindex
 from 
\begin_inset Formula $\mathcal{P}=\ints^{d}\times\mathcal{P}_{1}$
\end_inset

.
 The scatterers are located at positions 
\begin_inset Formula $\vect r_{\vect n,\alpha}=\vect R_{\vect n}+\vect r_{\alpha}$
\end_inset

 and their 
\begin_inset Formula $T$
\end_inset

-matrices are periodic, 
\begin_inset Formula $T_{\vect n,\alpha}=T_{\alpha}$
\end_inset

.
 In such system, the multiple-scattering problem 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Multiple-scattering problem"
plural "false"
caps "false"
noprefix "false"

\end_inset

 can be rewritten as
\end_layout

\begin_layout Standard
\begin_inset Formula 
\begin{equation}
\outcoeffp{\vect n,\alpha}-T_{\alpha}\sum_{\left(\vect m,\beta\right)\in\mathcal{P}\backslash\left\{ \left(\vect n,\alpha\right)\right\} }\tropsp{\vect n,\alpha}{\vect m,\beta}\outcoeffp{\vect m,\beta}=T_{\alpha}\rcoeffincp{\vect n,\alpha}.\quad\left(\vect n,\alpha\right)\in\mathcal{P}\label{eq:Multiple-scattering problem periodic}
\end{equation}

\end_inset


\end_layout

\begin_layout Standard
Due to periodicity, we can also write 
\begin_inset Formula $\tropsp{\vect n,\alpha}{\vect m,\beta}=\tropsp{\alpha}{\beta}\left(\vect R_{\vect m}-\vect R_{\vect n}\right)=\tropsp{\alpha}{\beta}\left(\vect R_{\vect m-\vect n}\right)=\tropsp{\vect 0,\alpha}{\vect m-\vect n,\beta}$
\end_inset

.
 Assuming quasi-periodic right-hand side with quasi-momentum 
\begin_inset Formula $\vect k$
\end_inset

, 
\begin_inset Formula $\rcoeffincp{\vect n,\alpha}=\rcoeffincp{\vect 0,\alpha}\left(\vect k\right)e^{i\vect k\cdot\vect R_{\vect n}}$
\end_inset

, the solutions of 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Multiple-scattering problem periodic"
plural "false"
caps "false"
noprefix "false"

\end_inset

 will be also quasi-periodic according to Bloch theorem, 
\begin_inset Formula $\outcoeffp{\vect n,\alpha}=\outcoeffp{\vect 0,\alpha}\left(\vect k\right)e^{i\vect k\cdot\vect R_{\vect n}}$
\end_inset

, and eq.
 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Multiple-scattering problem periodic"
plural "false"
caps "false"
noprefix "false"

\end_inset

 can be rewritten as follows
\begin_inset Formula 
\begin{align}
\outcoeffp{\vect 0,\alpha}\left(\vect k\right)e^{i\vect k\cdot\vect R_{\vect n}}-T_{\alpha}\sum_{\left(\vect m,\beta\right)\in\mathcal{P}\backslash\left\{ \left(\vect n,\alpha\right)\right\} }\tropsp{\vect n,\alpha}{\vect m,\beta}\outcoeffp{\vect 0,\beta}\left(\vect k\right)e^{i\vect k\cdot\vect R_{\vect m}} & =T_{\alpha}\rcoeffincp{\vect 0,\alpha}\left(\vect k\right)e^{i\vect k\cdot\vect R_{\vect n}},\nonumber \\
\outcoeffp{\vect 0,\alpha}\left(\vect k\right)-T_{\alpha}\sum_{\left(\vect m,\beta\right)\in\mathcal{P}\backslash\left\{ \left(\vect n,\alpha\right)\right\} }\tropsp{\vect 0,\alpha}{\vect m-\vect n,\beta}\outcoeffp{\vect 0,\beta}\left(\vect k\right)e^{i\vect k\cdot\vect R_{\vect m-\vect n}} & =T_{\alpha}\rcoeffincp{\vect 0,\alpha}\left(\vect k\right),\nonumber \\
\outcoeffp{\vect 0,\alpha}\left(\vect k\right)-T_{\alpha}\sum_{\left(\vect m,\beta\right)\in\mathcal{P}\backslash\left\{ \left(\vect 0,\alpha\right)\right\} }\tropsp{\vect 0,\alpha}{\vect m,\beta}\outcoeffp{\vect 0,\beta}\left(\vect k\right)e^{i\vect k\cdot\vect R_{\vect m}} & =T_{\alpha}\rcoeffincp{\vect 0,\alpha}\left(\vect k\right),\nonumber \\
\outcoeffp{\vect 0,\alpha}\left(\vect k\right)-T_{\alpha}\sum_{\beta\in\mathcal{P}}W_{\alpha\beta}\left(\vect k\right)\outcoeffp{\vect 0,\beta}\left(\vect k\right) & =T_{\alpha}\rcoeffincp{\vect 0,\alpha}\left(\vect k\right),\label{eq:Multiple-scattering problem unit cell}
\end{align}

\end_inset

so we reduced the initial scattering problem to one involving only the field
 expansion coefficients from a single unit cell, but we need to compute
 the 
\begin_inset Quotes eld
\end_inset

lattice Fourier transform
\begin_inset Quotes erd
\end_inset

 of the translation operator,
\begin_inset Formula 
\begin{equation}
W_{\alpha\beta}(\vect k)\equiv\sum_{\vect m\in\ints^{d}}\left(1-\delta_{\alpha\beta}\delta_{\vect m\vect 0}\right)\tropsp{\vect 0,\alpha}{\vect m,\beta}e^{i\vect k\cdot\vect R_{\vect m}},\label{eq:W definition}
\end{equation}

\end_inset

evaluation of which is possible but quite non-trivial due to the infinite
 lattice sum, so we explain it separately in Sect.
 
\begin_inset CommandInset ref
LatexCommand eqref
reference "subsec:W operator evaluation"
plural "false"
caps "false"
noprefix "false"

\end_inset

.
\end_layout

\begin_layout Standard
As in the case of a finite system, eq.
 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Multiple-scattering problem unit cell"
plural "false"
caps "false"
noprefix "false"

\end_inset

 can be written in a shorter block-matrix form,
\begin_inset Formula 
\begin{equation}
\left(I-TW\right)\outcoeffp{\vect 0}\left(\vect k\right)=T\rcoeffincp{\vect 0}\left(\vect k\right)\label{eq:Multiple-scattering problem unit cell block form}
\end{equation}

\end_inset

 Eq.
 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Multiple-scattering problem unit cell"
plural "false"
caps "false"
noprefix "false"

\end_inset

 can be used to calculate electromagnetic response of the structure to external
 quasiperiodic driving field – most notably a plane wave.
 However, the non-trivial solutions of the equation with right hand side
 (i.e.
 the external driving) set to zero, 
\begin_inset Formula 
\begin{equation}
\left(I-TW\right)\outcoeffp{\vect 0}\left(\vect k\right)=0,\label{eq:lattice mode equation}
\end{equation}

\end_inset

describes the 
\emph on
lattice modes.

\emph default
 Non-trivial solutions to 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:lattice mode equation"
plural "false"
caps "false"
noprefix "false"

\end_inset

 exist if the matrix on the left-hand side 
\begin_inset Formula $M\left(\omega,\vect k\right)=\left(I-T\left(\omega\right)W\left(\omega,\vect k\right)\right)$
\end_inset

 is singular – this condition gives the 
\emph on
dispersion relation
\emph default
 for the periodic structure.
 Note that in realistic (lossy) systems, at least one of the pair 
\begin_inset Formula $\omega,\vect k$
\end_inset

 will acquire complex values.
 The solution 
\begin_inset Formula $\outcoeffp{\vect 0}\left(\vect k\right)$
\end_inset

 is then obtained as the right 
\begin_inset Note Note
status open

\begin_layout Plain Layout
CHECK!
\end_layout

\end_inset

 singular vector of 
\begin_inset Formula $M\left(\omega,\vect k\right)$
\end_inset

 corresponding to the zero singular value.
\end_layout

\begin_layout Subsection
Numerical solution
\end_layout

\begin_layout Standard
In practice, equation 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Multiple-scattering problem unit cell block form"
plural "false"
caps "false"
noprefix "false"

\end_inset

 is solved in the same way as eq.
 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Multiple-scattering problem block form"
plural "false"
caps "false"
noprefix "false"

\end_inset

 in the multipole degree truncated form.
\end_layout

\begin_layout Standard
The lattice mode problem 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:lattice mode equation"
plural "false"
caps "false"
noprefix "false"

\end_inset

 is (after multipole degree truncation) solved by finding 
\begin_inset Formula $\omega,\vect k$
\end_inset

 for which the matrix 
\begin_inset Formula $M\left(\omega,\vect k\right)$
\end_inset

 has a zero singular value.
 A naïve approach to do that is to sample a volume with a grid in the 
\begin_inset Formula $\left(\omega,\vect k\right)$
\end_inset

 space, performing a singular value decomposition of 
\begin_inset Formula $M\left(\omega,\vect k\right)$
\end_inset

 at each point and finding where the lowest singular value of 
\begin_inset Formula $M\left(\omega,\vect k\right)$
\end_inset

 is close enough to zero.
 However, this approach is quite expensive, for 
\begin_inset Formula $W\left(\omega,\vect k\right)$
\end_inset

 has to be evaluated for each 
\begin_inset Formula $\omega,\vect k$
\end_inset

 pair separately (unlike the original finite case 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Multiple-scattering problem block form"
plural "false"
caps "false"
noprefix "false"

\end_inset

 translation operator 
\begin_inset Formula $\trops$
\end_inset

, which, for a given geometry, depends only on frequency).
 Therefore, a much more efficient but not completely robust approach to
 determine the photonic bands is to sample the 
\begin_inset Formula $\vect k$
\end_inset

-space (a whole Brillouin zone or its part) and for each fixed 
\begin_inset Formula $\vect k$
\end_inset

 to find a corresponding frequency 
\begin_inset Formula $\omega$
\end_inset

 with zero singular value of 
\begin_inset Formula $M\left(\omega,\vect k\right)$
\end_inset

 using a minimisation algorithm (two- or one-dimensional, depending on whether
 one needs the exact complex-valued 
\begin_inset Formula $\omega$
\end_inset

 or whether the its real-valued approximation is satisfactory).
 Typically, a good initial guess for 
\begin_inset Formula $\omega\left(\vect k\right)$
\end_inset

 is obtained from the empty lattice approximation, 
\begin_inset Formula $\left|\vect k\right|=\sqrt{\epsilon\mu}\omega/c_{0}$
\end_inset

 (modulo reciprocal lattice points
\begin_inset Note Note
status open

\begin_layout Plain Layout
TODO write this in a clean way
\end_layout

\end_inset

).
 A somehow challenging step is to distinguish the different bands that can
 all be very close to the empty lattice approximation, especially if the
 particles in the systems are small.
 In high-symmetry points of the Brilloin zone, this can be solved by factorising
 
\begin_inset Formula $M\left(\omega,\vect k\right)$
\end_inset

 into irreducible representations 
\begin_inset Formula $\Gamma_{i}$
\end_inset

 and performing the minimisation in each irrep separately, cf.
 Section 
\begin_inset CommandInset ref
LatexCommand ref
reference "sec:Symmetries"
plural "false"
caps "false"
noprefix "false"

\end_inset

, and using the different 
\begin_inset Formula $\omega_{\Gamma_{i}}\left(\vect k\right)$
\end_inset

 to obtain the initial guesses for the nearby points 
\begin_inset Formula $\vect k+\delta\vect k$
\end_inset

.
 Another, more robust approach is Beyn's contour integral algorithm 
\begin_inset CommandInset citation
LatexCommand cite
key "beyn_integral_2012"
literal "false"

\end_inset

 which finds the roots of 
\begin_inset Formula $M\left(\omega,\vect k\right)=0$
\end_inset

 in a given frequency contour.
\end_layout

\begin_layout Subsection
Computing the Fourier sum of the translation operator
\begin_inset CommandInset label
LatexCommand label
name "subsec:W operator evaluation"

\end_inset


\end_layout

\begin_layout Standard
The problem evaluating 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:W definition"

\end_inset

 is the asymptotic behaviour of the translation operator, 
\begin_inset Formula $\tropsp{\vect 0,\alpha}{\vect m,\beta}\sim\left|\vect R_{\vect m}\right|^{-1}e^{i\kappa\left|\vect R_{\vect m}\right|}$
\end_inset

 that does not in the strict sense converge for any 
\begin_inset Formula $d>1$
\end_inset

-dimensional lattice.
\begin_inset Note Note
status open

\begin_layout Plain Layout
\begin_inset Foot
status open

\begin_layout Plain Layout
Note that 
\begin_inset Formula $d$
\end_inset

 here is dimensionality of the lattice, not the space it lies in, which
 I for certain reasons assume to be three.
 (TODO few notes on integration and reciprocal lattices in some appendix)
\end_layout

\end_inset


\end_layout

\end_inset

 The problem of poorly converging lattice sums has been originally solved
 for electrostatic potentials with Ewald summation 
\begin_inset CommandInset citation
LatexCommand cite
key "ewald_berechnung_1921"
literal "false"

\end_inset

.
 Its basic idea is to decompose the divide the lattice-summed function in
 two parts: a short-range part that decays fast and can be summed directly,
 and a long-range part which decays poorly but is fairly smooth everywhere,
 so that its Fourier transform decays fast enough, and to deal with the
 long range part by Poisson summation over the reciprocal lattice.
 The same idea can be used also in this case case of linear electrodynamic
 problems, just the technical details are more complicated than in electrostatic
s.
\end_layout

\begin_layout Standard
In eq.
 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:translation operator singular"
plural "false"
caps "false"
noprefix "false"

\end_inset

 we demonstratively expressed the translation operator elements as linear
 combinations of (outgoing) scalar spherical wavefunctions 
\begin_inset Formula $\sswfoutlm lm\left(\vect r\right)=h_{l}^{\left(1\right)}\left(r\right)\ush lm\left(\uvec r\right)$
\end_inset

, because for them, fortunately, exponentially convergent Ewald-type summation
 formulae have been already developed 
\begin_inset Note Note
status open

\begin_layout Plain Layout
add refs
\end_layout

\end_inset

 
\begin_inset CommandInset citation
LatexCommand cite
key "moroz_quasi-periodic_2006,linton_one-_2009,linton_lattice_2010"
literal "false"

\end_inset

 and can be applied to our case.
 If we formally label 
\begin_inset Marginal
status open

\begin_layout Plain Layout
Check signs.
\end_layout

\end_inset


\begin_inset Formula 
\begin{equation}
\sigma_{l,m}\left(\vect k,\vect s\right)=\sum_{\vect n\in\ints^{d}}\left(1-\delta_{\vect{R_{n}},\vect s}\right)e^{i\vect{\vect k}\cdot\left(\vect R_{\vect n}-\vect s\right)}\sswfoutlm lm\left(\vect{R_{n}}-\vect s\right),\label{eq:sigma lattice sums}
\end{equation}

\end_inset

we see from eqs.
 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:translation operator singular"
plural "false"
caps "false"
noprefix "false"

\end_inset

,
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:W definition"
plural "false"
caps "false"
noprefix "false"

\end_inset

 that the matrix elements of 
\begin_inset Formula $W_{\alpha\beta}(\vect k)$
\end_inset

 read 
\begin_inset Note Note
status open

\begin_layout Plain Layout
\begin_inset Formula 
\[
W_{\alpha\beta}(\vect k)\equiv\sum_{\vect m\in\ints^{d}}\left(1-\delta_{\alpha\beta}\right)\tropsp{\vect 0,\alpha}{\vect m,\beta}e^{i\vect k\cdot\vect R_{\vect m}},
\]

\end_inset


\end_layout

\end_inset

 
\begin_inset Formula 
\begin{align*}
W_{\alpha,\tau lm;\beta,\tau l'm'}(\vect k) & =\sum_{\lambda=\left|l-l'\right|}^{l+l'}C_{lm;l'm'}^{\lambda}\sigma_{\lambda,m-m'}\left(\vect k,\vect r_{\beta}-\vect r_{\alpha}\right),\\
W_{\alpha,\tau lm;\beta,\tau'l'm'}(\vect k) & =\sum_{\lambda=\left|l-l'\right|+1}^{l+l'}D_{lm;l'm'}^{\lambda}\sigma_{\lambda,m-m'}\left(\vect k,\vect r_{\beta}-\vect r_{\alpha}\right),\quad\tau'\ne\tau,
\end{align*}

\end_inset


\begin_inset Marginal
status open

\begin_layout Plain Layout
Check signs
\end_layout

\end_inset

where the constant factors are exactly the same as in 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:translation operator constant factors"
plural "false"
caps "false"
noprefix "false"

\end_inset

.
\end_layout

\begin_layout Standard
For reader's reference, we list the Ewald-type formulae for lattice sums
 
\begin_inset Formula $\sigma_{l,m}\left(\vect k,\vect s\right)$
\end_inset

 from 
\begin_inset CommandInset citation
LatexCommand cite
key "linton_lattice_2010"
literal "false"

\end_inset

 rewritten in a way that is independent on particular phase or normalisation
 conventions of vector spherical harmonics.
\end_layout

\begin_layout Standard
In all three dimensionality cases, the lattice sums are divided into short-range
 and long-range parts, 
\begin_inset Formula $\sigma_{l,m}\left(\vect k,\vect s\right)=\sigma_{l,m}^{\left(\mathrm{S},\eta\right)}\left(\vect k,\vect s\right)+\sigma_{l,m}^{\left(\mathrm{L},\eta\right)}\left(\vect k,\vect s\right)$
\end_inset

 depending on a positive parameter 
\begin_inset Formula $\eta$
\end_inset

.
 The short-range part has in all three cases the same form:
\begin_inset Note Note
status open

\begin_layout Plain Layout
Check sign of s
\end_layout

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Formula 
\begin{multline}
\sigma_{l,m}^{\left(\mathrm{S},\eta\right)}\left(\vect k,\vect s\right)=-\frac{2^{l+1}i}{\kappa^{l+1}\sqrt{\pi}}\sum_{\vect n\in\ints^{d}}\left(1-\delta_{\vect{R_{n}},-\vect s}\right)\left|\vect{R_{n}+\vect s}\right|^{l}\ush lm\left(\vect{R_{n}+\vect s}\right)e^{i\vect k\cdot\left(\vect{R_{n}+\vect s}\right)}\int_{\eta}^{\infty}e^{-\left|\vect{R_{n}+\vect s}\right|^{2}\xi^{2}}e^{-\kappa/4\xi^{2}}\xi^{2l}\ud\xi\\
+\frac{\delta_{l0}\delta_{m0}}{\sqrt{4\pi}}\Gamma\left(-\frac{1}{2},-\frac{\kappa}{4\eta^{2}}\right)\ush lm\left(\vect{R_{n}+\vect s}\right),\label{eq:Ewald in 3D short-range part}
\end{multline}

\end_inset


\begin_inset Note Note
status open

\begin_layout Plain Layout
NEPATŘÍ TAM NĚJAKÁ DELTA FUNKCE K PŮVODNÍMU 
\begin_inset Formula $\sigma_{n}^{m(0)}$
\end_inset

?
\end_layout

\end_inset


\begin_inset Note Note
status open

\begin_layout Plain Layout
Poznámka ohledně zahození radiální části u kulových fcí?
\end_layout

\end_inset

The long-range part for cases 
\begin_inset Formula $d=1,2$
\end_inset

 reads
\begin_inset Note Note
status open

\begin_layout Plain Layout
check sign of 
\begin_inset Formula $\vect k$
\end_inset


\end_layout

\end_inset


\begin_inset Formula 
\begin{multline}
\sigma_{l,m}^{\left(\mathrm{S},\eta\right)}\left(\vect k,\vect s\right)=-\frac{i^{l+1}}{\kappa^{d}\mathcal{A}}\pi^{2+\left(3-d\right)/2}2\left(\left(l-m\right)/2\right)!\left(\left(l+m\right)/2\right)!\times\\
\times\sum_{\vect K\in\Lambda^{*}}\ush lm\left(\vect k+\vect K\right)\sum_{j=0}^{\left[\left(l-\left|m\right|/2\right)\right]}\frac{\left(-1\right)^{j}\left(\left|\vect k+\vect K\right|/\kappa\right)^{l-2j}\Gamma\left(-j,\frac{k^{2}\gamma\left(\left|\vect k+\vect K\right|/\kappa\right)}{4\eta^{2}}\right)}{j!\left(\frac{1}{2}\left(l-m\right)-j\right)!\left(\frac{1}{2}\left(l+m\right)-j\right)!}\left(\gamma\left(\left|\vect k+\vect K\right|/\kappa\right)\right)^{2j+3-d}\label{eq:Ewald in 3D long-range part 1D 2D}
\end{multline}

\end_inset

and for 
\begin_inset Formula $d=3$
\end_inset


\begin_inset Formula 
\begin{equation}
\sigma_{l,m}^{\left(\mathrm{S},\eta\right)}\left(\vect k,\vect s\right)=\frac{4\pi i^{l+1}}{\kappa\mathcal{A}}\sum_{\vect K\in\Lambda^{*}}\frac{\left(\left|\vect k+\vect K\right|/\kappa\right)^{l}}{\kappa^{2}-\left|\vect k+\vect K\right|^{2}}e^{\left(\kappa^{2}-\left|\vect k+\vect K\right|^{2}\right)/4\eta^{2}}\ush lm\left(\vect k+\vect K\right).\label{eq:Ewald in 3D long-range part 3D}
\end{equation}

\end_inset

Here 
\begin_inset Formula $\mathcal{A}$
\end_inset

 is the unit cell volume (or length/area in the 1D/2D lattice cases).
 The sums are taken over the reciprocal lattice 
\begin_inset Formula $\Lambda^{*}$
\end_inset

 with lattice vectors 
\begin_inset Formula $\left\{ \vect b_{i}\right\} _{i=1}^{d}$
\end_inset

 satisfying 
\begin_inset Formula $\vect a_{i}\cdot\vect b_{j}=\delta_{ij}$
\end_inset

.
 The function 
\begin_inset Formula $\gamma\left(z\right)$
\end_inset

 used in 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Ewald in 3D long-range part 1D 2D"
plural "false"
caps "false"
noprefix "false"

\end_inset

 is defined as
\begin_inset Formula 
\[
\gamma\left(z\right)=\left(z-1\right)^{\frac{1}{2}}\left(z+1\right)^{\frac{1}{2}},\quad-\frac{3\pi}{2}<\arg\left(z-1\right)<\frac{\pi}{2},-\frac{\pi}{2}<\arg\left(z+1\right)<\frac{3\pi}{2}.
\]

\end_inset

The Ewald parameter 
\begin_inset Formula $\eta$
\end_inset

 determines the pace of convergence of both parts.
 The larger 
\begin_inset Formula $\eta$
\end_inset

 is, the faster 
\begin_inset Formula $\sigma_{l,m}^{\left(\mathrm{S},\eta\right)}\left(\vect k,\vect s\right)$
\end_inset

 converges but the slower 
\begin_inset Formula $\sigma_{l,m}^{\left(L,\eta\right)}\left(\vect k,\vect s\right)$
\end_inset

 converges.
 Therefore (based on the lattice geometry) it has to be adjusted in a way
 that a reasonable amount of terms needs to be evaluated numerically from
 both 
\begin_inset Formula $\sigma_{l,m}^{\left(\mathrm{S},\eta\right)}\left(\vect k,\vect s\right)$
\end_inset

 and 
\begin_inset Formula $\sigma_{l,m}^{\left(\mathrm{L},\eta\right)}\left(\vect k,\vect s\right)$
\end_inset

 .
\begin_inset Marginal
status open

\begin_layout Plain Layout
What would be a good choice?
\end_layout

\end_inset

 
\begin_inset Marginal
status open

\begin_layout Plain Layout
I have some error estimates derived in my notes.
 Should I include them?
\end_layout

\end_inset


\begin_inset Note Note
status open

\begin_layout Plain Layout
Generally, a good choice for 
\begin_inset Formula $\eta$
\end_inset

 is TODO; in order to achieve accuracy TODO, one has to evaluate the terms
 on TODO lattice points.
\end_layout

\end_inset

 
\begin_inset Note Note
status open

\begin_layout Plain Layout
(I HAVE SOME DERIVATIONS OF THE ESTIMATES IN MY NOTES; SHOULD I INCLUDE
 THEM?)
\end_layout

\end_inset


\end_layout

\begin_layout Standard
In practice, the integrals in 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Ewald in 3D short-range part"
plural "false"
caps "false"
noprefix "false"

\end_inset

 can be easily evaluated by numerical quadrature and the incomplete 
\begin_inset Formula $\Gamma$
\end_inset

-functions using the series 8.7.3 from 
\begin_inset CommandInset citation
LatexCommand cite
key "NIST:DLMF"
literal "false"

\end_inset

.
\end_layout

\end_body
\end_document