Solving electrostatics problems with scuffstatic
scuffstatic is a tool within the scuffem code suite for solving a broad class of electrostatics problems.
The various calculations that scuffstatic may be divided into two categories: (1) calculations that yield the response of the geometry to one or more specific excitation stimuli ("type1" calculations), and (2) calculations that yield intrinsic properties of the material geometry, independent of any external excitation ("type2" calculations).
Type1 calculations
For type1 calculations, you pass commandline options to scuffstatic that specify a specific electrostatic "excitation." A single excitation consists of one or both of the following:

a set of electrostatic potential values at which to maintain the conductors in the geometry (option
potfile
); conductors for which no value is specified are maintained at 0V by default. 
a description of an external electrostatic field, which may be a superposition of
 the field of one or point charges (
monopole
),  the field of one or point dipoles (
dipole
),  a spatiallyconstant electrostatic field (
ConstField
)  an spatiallyvarying electrostatic potential defined by a
userspecified functional form (
PhiExt
).
 the field of one or point charges (
You may also use the ExcitationFile
option to
specify a list of excitations, each consisting of one or both
of the above ingredients. This is equivalent to running
scuffstatic multiple times
with options specifying a single excitation
(potfile
, monopole
, etc.) each timebut is much faster,
because of the structure of BEM solvers: once we have
done the setup work needed to solve for one
excitation (namely, forming and factorizing the
BEM matrix), solving for any number
of additional excitations is essentially "free" computationally.
Having specified one or more excitations for a type1 calculation, you may request multiple types of output, which will be computed separately for each excitation:

Values of the electrostatic potential and electric field components at arbitrary userspecified Evaluation Points inside or outside material bodies, output in the form of numerical data in a text file (option
EPFile
) 
Field visualization files plotting the potential and fields on one or more arbitrary userspecified FieldVisualization mesh(es) (option
FVMesh
), with the fieldvisualization mesh(es) optionally subject to one or more geometric transformations (optionFVMeshTransFile
) 
Surfacecharge visualization files showing the distribution of electrostatic surface charges induced by the excitation (option
PlotFile
)
Type2 calculations
In contrast to type1 calculations, type2 calculations do not involve any userspecified excitation. The available type2 calculations are:

Computation of the capacitance matrix for the conductors in the geometry (option
capfile
) 
Computation of the DC polarizabilities of the bodies (both conductors and dielectrics) in the geometry (option
polfile
) 
Computation of the Cmatrix, a sort of electrostatic version of the "Tmatrix" used to characterize the scattering properties of bodies at nonzero frequencies (options
CMatrixFile
and/orCMatrixHDF5File
). The Cmatrix was shown in this paper to be related to quantummechanical entanglement entropy.
Geometrical transformations
For both type1 and type2 calculations, you may use
the optional transfile
option to specify a list
of geometric transformations
to be applied to the scattering geometry; you will then
get back results for each specified output quantity
under each transformation.
Implicit dielectric substrates
You may use the optional substrate
option to specify
that your geometry exists in the presence of
a substrate consisting
of one or more (infinitearea) stacked dielectric layers with an
optional ground plane. This substrate will be treated implicitly
by scuffstatic; your .scuffgeo
file
and the .msh
files it specifies need only define the
(finitearea) conducting and/or dielectric objects
that lie above (or within the substrate.
Under the hood: Implementation of scuffstatic
As a technical detail, we note that the implementation of scuffstatic actually differs in some significant ways from the other codes in the scuffem suite; in particular, as compared to the scuffem core library, scuffstatic uses different basis functions and a different formulation of the boundaryelement method, as appropriate for zerofrequency problems. (More specifically, scuffstatic expands surface electric charge densities on PEC and dielectric surfaces using ``pulse'' basis functions, which are constant on individual triangles and vanishing everywhere else.) However, from the implementation standpoint, it turns out that the calculations needed to implement the electrostatics calculations in scuffstatic are a proper subset of the calculations already implemented in scuffem. Moreover, from the user's standpoint, the work needed to set up a scuffstatic problem (create surface meshes, write geometry files, etc.) is similar to the setup needed for the nonzerofrequency codes in the scuffem suite. This is why it makes sense to package these codes together.
Here is a brief technical memo discussing the implementation of scuffstatic, including both the underlying BEM electrostatics formulation and the execution of the various types of calculation (capacitance, polarizability, etc.) that the code can do.
1. scuffstatic commandline options
Options defining the geometry
geometry MyGeometry.scuffgeo substrate MySubstrate.substrate TransFile Transformations.trans
The mandatory geometry
option specifies a
scuffem geometry file
defining one or more surface meshes that comprise
your geometry.
The optional substrate
option specifies an
file describing a substrate consisting of zero or
more dielectric layers stacked atop an optional
ground plane; see here
for an example.
The optional TransFile
option specifies a list
of geometric transformations
to be applied to your geometry; each output calculation
you request will be repeated once for each
geometric transformation.
Options defining individual excitations for type1 calculations
To run a "type1" calculation with just a single excitation, you may specify any combination of the following options.
PotFile MyPotFile ConstField Ex Ey Ez Monopole x y z Q Dipole x y z Px Py Pz PhiExt PhiExpression(x,y,z)
Here MyPotFile
should be a simple text file
consisting of a list of (surface label, potential value) pairs,
where the surface label is the label assigned to a surface
in the .scuffgeo
file. For example, if your geometry
contains conductors labeled UpperSurface
and LowerSurface
,
which you wish to maintain at 1.2 volts and 3.4 volts
respectively, then MyPotFile
would look like this:
UpperSurface 1.2
LowerSurface 3.4
Any conductors that are not specified in the PotFile
are maintained at 0 volts by default.
ConstField Ex Ey Ez
specifies a spatiallyconstant
external electrostatic field with components
E=(Ex
,Ey
,Ez
).
Monopole x y z Q
specifies a point charge of strength Q
at cartesian coordinates (x
,y
,z
).
Dipole x y z px py pz
specifies a point dipole
at cartesian coordinates (x
,y
,z
)
with dipole moment p=(px
,py
,pz
).
PhiExt MyPhiExpression(x,y,z)
specifies a spatiallyvarying
electrostatic potential defined by the given function of cartesian
coordinates x
, y
, z
. Example: PhiExt cos(3*x)*cosh(3*y)
Options defining multiple excitations for type1 calculations
ExcitationFile MyExcitationFile
Specifies that MyExcitationFile
contains a list of
excitations. The file should consist of one or more
clauses of the form EXCITATION Label ... ENDEXCITATION
where Label
is an arbitrary label you assign to the
excitation (which will be used to tag the corresponding
output data). Each clause should contain one or more
lines, each defining either (a) a {ConductorLabel, PhiValue}
pair, or (b) an externalfield specification.
(Note that all externalfield specifications defined within
a single EXCITATION
clause are present simultaneously
when that excitation is active.)
Here's an example of an excitation file for a geometry
containing conductors labeled UpperSurface
and LowerSurface
:
EXCITATION UpperOnly UpperSurface 1 ENDEXCITATION EXCITATION LowerOnly LowerSurface 1 ENDEXCITATION EXCITATION Both UpperSurface 1 LowerSurface 1 ENDEXCITATION EXCITATION BothWithMonopoles UpperSurface 1 LowerSurface 1 Monopole 0 0 +1 1 Monopole 0 0 1 1 ENDEXCITATION EXCITATION Ex CONSTFIELD 1.0 0.0 0.0 ENDEXCITATION EXCITATION EyAndDipole CONSTFIELD 0.0 1.0 0.0 DIPOLE 0.1 0.2 0.3 0.4 0.5 0.2 ENDEXCITATION
Options defining outputs for type1 calculation
EPFile MyEPFile
Requests that the electrostatic potential and field
components be computed at each evaluation point
listed in the file MyEPFile
, which is a simple text
file containing three numbers per line (the coordinates
of a single evaluation point.) The resulting output file
will contain a header explaining how to interpret its contents.
FVMesh MyFVMesh.msh FVMeshTransFile MyFVMeshTransFile
Requests that electrostatic fields be computed
on a FieldVisualization mesh described by the
gmsh mesh
file MyFVMesh.msh.
This will produce
in an output file with extension .pp
that can be opened directly
in gmsh to visualize the fields in any region of space.
The optional FVMeshTransFile
defines a list of
geometric transformations to be applied
to MyFVMesh.msh.
This allows you to obtain visualization data on a
large region foliated by multiple translated or rotated copies of a
single mesh screen.
PlotFile MyPlotFile.pp
Requests creation of a gmsh visualization file named MyPlotFile.pp
plotting the induced charge density on all conducting and
dielectric surfaces in the geometry.
Options requesting outputs for type2 calculations
CapFile MyCapacitanceMatrix.dat
If you specify a file name using CapFile
, scuffstatic
will compute the full capacitance matrix for your geometry
and write the data to the specified file. (The file will be
overwritten if it already exists.)
PolFile MyPolFile.dat
If you specify a file name using PolFile
, scuffstatic
will compute the DC polarizability of each object in your
geometry and write the data to the specified file. (The file
will be overwritten if it already exists.)