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207 lines
7.4 KiB
TeX
207 lines
7.4 KiB
TeX
%
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% $Id$
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%
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\label{sec:ccsd}
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The NWChem coupled cluster energy module is primarily the work of
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Alistair Rendell and Rika Kobayashi, with contributions from David
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Bernholdt.
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The coupled cluster code can perform calculations with full iterative
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treatment of single and double excitations and non-iterative inclusion
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of triple excitation effects. It is presently limited to closed-shell
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(RHF) references.
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{\em Note that symmetry is not used within most of the CCSD(T) code.}
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This can have a profound impact on performance since the speed-up from
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symmetry is roughly the square of the number of irreducible
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representations. In the absence of symmetry, the performance of this
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code is competitive with other programs.
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The operation of the coupled cluster code is controlled by the input
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block
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\begin{verbatim}
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CCSD
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[MAXITER <integer maxiter default 20>]
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[THRESH <real thresh default 10e-6>]
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[TOL2E <real tol2e default min(10e-12 , 0.01*$thresh$)>]
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[DIISBAS <integer diisbas default 5>]
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[NODISK]
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[FREEZE [[core] (atomic || <integer nfzc default 0>)] \
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[virtual <integer nfzv default 0>]]
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[IPRT <integer IPRT default 0>]
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[PRINT ...]
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[NOPRINT ...]
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[NODISK]
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END
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\end{verbatim}
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Note that the keyword \verb+CCSD+ is used for the input block
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regardless of the actual level of theory desired (specified with the
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\verb+TASK+ directive). The following directives are recognized
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within the \verb+CCSD+ group.
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\section{{\tt MAXITER} --- Maximum number of iterations}
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The maximum number of iterations is set to 20 by default. This should
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be quite enough for most calculations, although particularly
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troublesome cases may require more.
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\begin{verbatim}
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MAXITER <integer maxiter default 20>
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\end{verbatim}
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\section{{\tt THRESH} --- Convergence threshold}
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Controls the convergence threshold for the iterative part of the
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calculation. Both the RMS error in the amplitudes {\em and} the
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change in energy must be less than {\tt thresh}.
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\begin{verbatim}
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THRESH <real thresh default 10e-6>
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\end{verbatim}
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\section{{\tt TOL2E} --- integral screening threshold}
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\begin{verbatim}
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TOL2E <real tol2e default min(10e-12 , 0.01*$thresh$)>
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\end{verbatim}
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The variable \verb+tol2e+ is used in determining the integral
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screening threshold for the evaluation of the energy and related
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quantities.
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{\em CAUTION!} At the present time, the \verb+tol2e+ parameter only
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affects the three- and four-virtual contributions, and the triples,
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all of which are done ``on the fly''. The transformations
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used for the other parts of the code currently have a hard-wired
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threshold of $10^{-12}$. The default for \verb+tol2e+ is set to match
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this, and since user input can only make the threshold smaller,
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setting this parameter can only make calculations take longer.
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\section{{\tt DIISBAS} --- DIIS subspace dimension}
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Specifies the maximum size of the subspace used in DIIS convergence
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acceleration. Note that DIIS requires the amplitudes and errors be
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stored for each iteration in the subspace. Obviously this can
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significantly increase memory requirements, and could force the user
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to reduce \verb+DIISBAS+ for large calculations.
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{\em Measures to alleviate this problem, including more compact
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storage of the quantities involved, and the possibility of disk
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storage are being considered, but have not yet been implemented.}
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\begin{verbatim}
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DIISBAS <integer diisbas default 5>
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\end{verbatim}
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\section{{\tt NODISK} --- disable integral caching}
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Disables integral caching on disk during the CCSD iterations. The effect
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of this keywork upon performance varies with the chemical system, basis
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set and hardware parameters. If the time for the first CCSD iteration
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exceeds the time for subsequent iterations, this may be indication that
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writing to disk is not useful and that \verb+NODISK+ will improve
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performance.
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\begin{verbatim}
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NODISK
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\end{verbatim}
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\section{{\tt FREEZE} --- Freezing orbitals}
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\begin{verbatim}
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[FREEZE [[core] (atomic || <integer nfzc default 0>)] \
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[virtual <integer nfzv default 0>]]
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\end{verbatim}
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This directive is idential to that used in the MP2 module, Section
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\ref{mp2:core}.
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\section{{\tt IPRT} --- Debug printing}
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This directive controls the level of output from the code, mostly to
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facilitate debugging and the like. The larger the value, the more
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output printed. From looking at the source code, the interesting
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values seem to be \verb+IPRT+ $>$ 5, 10, and 50.
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\begin{verbatim}
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IPRT <integer IPRT default 0>
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\end{verbatim}
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\section{PRINT and NOPRINT}
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The coupled cluster module supports the standard NWChem print control
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keywords, although very little in the code is actually hooked into
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this mechanism yet.
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\begin{tabular}{lll}
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\hline\hline
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Item & Print Level & Description \\
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\hline
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``reference'' & high & Wavefunction information\\
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``guess pair energies'' & debug & MP2 pair energies\\
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``byproduct energies'' & default & Intermediate energies \\
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``term debugging switches'' & debug & Switches for individual terms \\
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\hline\hline
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\end{tabular}
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\section{NODISK}
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Turns off disk based storing of 2-electron integrals during CCSD
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iterations (on by default).
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\section{Methods (Tasks) Recognized}
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Currently available methods are
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\begin{itemize}
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\item \verb+CCSD+ -- Full iterative inclusion of single and double
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excitations
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\item \verb=CCSD+T(CCSD)= -- The fourth order triples contribution computed with
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converged singles and doubles amplitudes
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\item \verb=CCSD(T)= -- The linearized triples approximation due to
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Raghavachari.
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\end{itemize}
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The calculation is invoked using the the \verb+TASK+ directive, so to
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perform a CCSD+T(CCSD) calculation, for example, the input file should
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include the directive
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\begin{verbatim}
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TASK CCSD+T(CCSD)
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\end{verbatim}
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Lower-level results which come as by-products (such as MP3/MP4) of the
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requested calculation are generally also printed in the output file
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and stored on the run-time database, but the method specified in the
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\verb+TASK+ directive is considered the primary result.
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\section{Debugging and Development Aids}
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The information in this section is intended for use by experts (both
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with the methodology and with the code), primarily for debugging and
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development work. Messing with stuff in listed in this section will
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probably make your calculation quantitatively {\em\bf wrong}\/!
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Consider yourself warned!
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\subsection{Switching On and Off Terms}
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The \verb+/DEBUG/+ common block contains a number of arrays which
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control the calculation of particular terms in the program. These are
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15-element integer arrays (although from the code only a few elements
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actually effect anything) which can be set from the input deck. See
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the code for details of how the arrays are interpreted.
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Printing of this data at run-time is controlled by the
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\verb+"term debugging switches"+ print option. The values are checked against
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the defaults at run-time and a warning is printed to draw attention to
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the fact that the calculation does not correspond precisely to the
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requested method.
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\begin{verbatim}
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DOA <integer array default 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2>
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DOB <integer array default 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2>
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DOG <integer array default 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1>
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DOH <integer array default 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1>
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DOJK <integer array default 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2>
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DOS <integer array default 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1>
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DOD <integer array default 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1>
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\end{verbatim}
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