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68 lines
2.3 KiB
TeX
68 lines
2.3 KiB
TeX
%
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% $Id$
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%
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\label{sec:vscf}
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The VSCF module can be used to calculate the anharmonic contributions to the
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vibrational modes of the molecule of interest. Energies are calculated on a
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one-dimensional grid along each normal mode, on a two-dimensional grid along
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each pair of normal modes, and optionally on a three-dimensional grid along
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each triplet of normal modes. These energies are then used to calculate the
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vibrational nuclear wavefunction at an SCF- (VSCF) and MP2-like (cc-VSCF) level
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of theory.
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VSCF can be used at all levels of theory, SCF and correlated methods, and DFT.
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For correlated methods, only the SCF level dipole is evaluated and used to
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calculate the IR intensity values.
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The VSCF module is started when the task directive
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\verb+TASK <theory> vscf+ is defined in the user input file. The input
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format has the form:
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\begin{verbatim}
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VSCF
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[coupling <string couplelevel default "pair">]
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[ngrid <integer default 16 >]
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[iexcite <integer default 1 >]
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[vcfct <real default 1.0>]
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END
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\end{verbatim}
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The order of coupling of the harmonic normal modes included in the calculation
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is controlled by the specifying:
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\begin{verbatim}
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coupling <string couplelevel default "pair">
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\end{verbatim}
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For \verb+coupling=diagonal+ a one-dimensional grid along each normal mode is computed.
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For \verb+coupling=pair+ a two-dimensional grid along each pair of normal modes is computed.
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For \verb+coupling=triplet+ a three-dimensional grid along each triplet of normal modes is computed.
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The number of grid points along each normal mode, or pair of modes can be defined
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by specifying:
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\begin{verbatim}
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ngrid <integer default 16>
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\end{verbatim}
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This VSCF module by default calculates the ground state (v=0), but can also calculate excited
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states (such as v=1). The number of excited states calculated is defined by specifying:
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\begin{verbatim}
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iexcite <integer default 1>
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\end{verbatim}
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With \verb+iexcite=1+ the fundamental frequencies are calculated.
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With \verb+iexcite=2+ the first overtones are calculated.
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With \verb+iexcite=3+ the second overtones are calculated.
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In certain cases the pair coupling potentials can become larger than those for a single
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normal mode. In this case the pair potentials need to be scaled down. The scaling factor
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used can be defined by specifying:
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\begin{verbatim}
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vcfct <real default 1.0>
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\end{verbatim}
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