\documentclass{article}
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\title{ADMBase}
\author{Tom Goodale}
\date{April 2002}
\maketitle

\abstract{Provides the basic ADM variables used in the $3+1$ formalism}

\section{Purpose}

This thorn provides the basic variables used to communicate between
thorns doing General Relativity in the $3+1$ formalism.

The variables are:
\begin{itemize}
\item
The metric tensor -- {\bf gxx}, {\bf gxy}, {\bf gxz},{\bf gyy},{\bf gyz},{\bf gzz}
\item
The extrinsic curvature tensor -- {\bf kxx}, {\bf kxy}, {\bf kxz},{\bf kyy},{\bf kyz},{\bf kzz}
\item
The lapse -- {\bf alp}
\item
The shift -- {\bf betax},{\bf betay},{\bf betaz}
\end{itemize}

By default the metric and extrinsic curvature tensors are assumed to
be the physical ones, however these semantics can be changed by use of
the {\bf metric\_type} parameter.  ADMBase provides the default value
of `physical', however another thorn could extend this with a value
stating that the metric or curvature have some conformal factor or
whatever.  This means that all thorns which inherit from ADMBase
should check the value of {\bf metric\_type} in a routine scheduled at
{\bf CCTK\_PARAMCHECK} and signal an error if the thorn does not
recognise the value.

ADMBase allocates one timelevel of memory for all variables, except the
shift, which is only allocated if the {\bf initial\_shift} parameter is
set to a value other than `none'.  The state of the shift storage is
indicated by the {\bf shift\_state} grid scalar.  This is 1 if there
is storage for the shift, and 0 otherwise.

The thorn provides, on request, initial data to set the metric and
extrinsic curvature to flat space in cartesian coordinates, to set the
initial lapse to one and the initial shift to zero.

\section{Comments}



% Automatically created from the ccl files by using gmake thorndoc
\include{interface}
\include{param}
\include{schedule}

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