mirror of
https://github.com/nwchemgit/nwchem.git
synced 2026-07-28 06:05:44 -04:00
88 lines
3.2 KiB
TeX
88 lines
3.2 KiB
TeX
\label{sec:constraintsnew}
|
|
|
|
The constraints directive allows the user to specify which constraints
|
|
should be imposed on the system during the geometry optimization. Currently
|
|
such constraints are limited to fixed atom positions and
|
|
harmonic restraints (springs) on the distance between the two atoms. The
|
|
general form of constraints block is presented below:
|
|
\begin{verbatim}
|
|
CONSTRAINTS [string name ] \
|
|
[clear] \
|
|
[enable||disable] \
|
|
[fix atom <integer list>] \
|
|
[spring bond <integer atom1> <integer atom2> <real k> <real r0> ]
|
|
END
|
|
|
|
\end{verbatim}
|
|
The keywords are described below
|
|
\begin{itemize}
|
|
\item {\it name} -- optional keyword that associates a name with a given set of
|
|
constraints. Any unnamed set of constraints will be given a name ''default''
|
|
and will be automatically loaded prior to a calculation. Any constraints
|
|
with the name other than ''default'' will have to be loaded manually using
|
|
SET\ directive. For example,
|
|
\end{itemize}
|
|
|
|
\begin{verbatim}
|
|
CONSTRAINTS one
|
|
spring bond 1 3 5.0 1.3
|
|
fix atom 1
|
|
END
|
|
|
|
#the above constraints can be loaded using set directive
|
|
set constraints one
|
|
....
|
|
task ....
|
|
\end{verbatim}
|
|
|
|
\begin{itemize}
|
|
\item {\it clear} -- destroys any prior constraint information. This may be
|
|
useful when the same constraints have to be redefined or completely removed from
|
|
the runtime database.
|
|
|
|
\item {\it enable||disable} -- enables or
|
|
disables particular set of constraints without actually removing the
|
|
information from the runtime database.
|
|
|
|
\item {\it fix atom} -- fixes atom positions during geometry optimization. This
|
|
directive requires an integer list that specifies which atoms are to be
|
|
fixed. This directive can be repeated within a given constraints block.
|
|
To illustrate the use "{\it fix atom}" directive let us
|
|
consider a situation where we would like to fix atoms 1, 3, 4, 5, 6 while performing an optimization on some hypothetical system.
|
|
There are actually several
|
|
ways to enter this particular constraint.
|
|
There is a straightforward option which requires the most typing
|
|
|
|
\begin{verbatim}
|
|
constraints
|
|
fix atom 1 3 4 5 6
|
|
end
|
|
\end{verbatim}
|
|
|
|
Second method uses list input
|
|
\begin{verbatim}
|
|
constraints
|
|
fix atom 1 3:6
|
|
end
|
|
\end{verbatim}
|
|
|
|
Third approach illustrates the use of multiple fix atom directives
|
|
\begin{verbatim}
|
|
constraints
|
|
fix atom 1
|
|
fix atom 3:6
|
|
end
|
|
\end{verbatim}
|
|
|
|
\item {\it spring} bond <$i j k r_0$> -- places a spring with a spring constant $k$ and equilibrium length $r_0$
|
|
between atoms $i$ and $j$ (all in atomic units). Please note that this type of constraint adds an additional term to
|
|
the total energy expression
|
|
\[
|
|
E=E_{total}+\frac{1}{2}k(r_{ij}-r_0)^2
|
|
\]
|
|
This additional term forces the distance between atoms $i$ and $j$ to be in the vicinity of $r_0$ but never exactly that. In general
|
|
the spring energy term will always have some nonzero residual value, and this has to be accounted for when comparing total
|
|
energies. The "{\it spring bond}" directive can be repeated within a given constraints block. If the spring between the same pair of atoms
|
|
is defined more than once, it will be replaced by the latest specification in the order it appears in the input block.
|
|
\end{itemize}
|
|
|