The theory of correlation functions is well established~\cite{Allen87}.
We describe here the implementation of the various
\normindex{correlation} function flavors in the {\gromacs} code.
-The definition of the \index{autocorrelation function} (ACF)
+The definition of the autocorrelation function\index{autocorrelation function}
+(ACF)
$C_f(t)$ for a property $f(t)$ is:
\beq
C_f(t) ~=~ \left\langle f(\xi) f(\xi+t)\right\rangle_{\xi}
\section{Mean Square Displacement}
\label{sec:msd}
{\tt gmx msd}\\
-To determine the self \index{diffusion coefficient} $D_A$ of
+To determine the self diffusion coefficient\index{diffusion coefficient} $D_A$
+of
particles of type $A$, one can use the \normindex{Einstein
relation}~\cite{Allen87}:
\beq
A force field is built up from two distinct components:
\begin{itemize}
\item The set of equations (called the {\em
- \index{potential function}s}) used to generate the potential
+potential functions}\index{potential function}) used to generate the potential
energies and their derivatives, the forces. These are described in
detail in the previous chapter.
\item The parameters used in this set of equations. These are not
%
% This file is part of the GROMACS molecular simulation package.
%
-% Copyright (c) 2013,2014, by the GROMACS development team, led by
+% Copyright (c) 2013,2014,2015, by the GROMACS development team, led by
% Mark Abraham, David van der Spoel, Berk Hess, and Erik Lindahl,
% and including many others, as listed in the AUTHORS file in the
% top-level source directory and at http://www.gromacs.org.
from the inner loop~\cite{Bekker93b}.
\subsection{Virial}
-In a system with \index{periodic boundary conditions}, the
+In a system with periodic boundary conditions\index{periodic boundary
+conditions}, the
periodicity must be taken into account for the virial:
\beq
\Xi~=~-\half~\sum_{i < j}^{N}~\rnij\otimes\Fvij
\subsection{User-specified potential functions}
\label{subsec:userpot}
-You can also use your own \index{potential function}s
-without editing the {\gromacs} code.
-The potential function should be according to the following equation
+You can also use your own potential functions\index{potential function} without
+editing the {\gromacs} code. The potential function should be according to the
+following equation
\beq
V(r_{ij}) ~=~ \frac{q_i q_j}{4 \pi\epsilon_0} f(r_{ij}) + C_6 \,g(r_{ij}) + C_{12} \,h(r_{ij})
\eeq