1%
2% $Id$
3%
4\label{sec:vscf}
5
6The VSCF module can be used to calculate the anharmonic contributions to the
7vibrational modes of the molecule of interest. Energies are calculated on a
8one-dimensional grid along each normal mode, on a two-dimensional grid along
9each pair of normal modes, and optionally on a three-dimensional grid along
10each triplet of normal modes. These energies are then used to calculate the
11vibrational nuclear wavefunction at an SCF- (VSCF) and MP2-like (cc-VSCF) level
12of theory.
13
14VSCF can be used at all levels of theory, SCF and correlated methods, and DFT.
15For correlated methods, only the SCF level dipole is evaluated and used to
16calculate the IR intensity values.
17
18The VSCF module is started when the task directive
19\verb+TASK <theory> vscf+ is defined in the user input file. The input
20format has the form:
21
22\begin{verbatim}
23  VSCF
24    [coupling <string couplelevel default "pair">]
25    [ngrid    <integer default 16 >]
26    [iexcite  <integer default 1  >]
27    [vcfct    <real    default 1.0>]
28  END
29\end{verbatim}
30
31The order of coupling of the harmonic normal modes included in the calculation
32is controlled by the specifying:
33
34\begin{verbatim}
35    coupling <string couplelevel default "pair">
36\end{verbatim}
37
38For \verb+coupling=diagonal+ a one-dimensional grid along each normal mode is computed.
39For \verb+coupling=pair+ a two-dimensional grid along each pair of normal modes is computed.
40For \verb+coupling=triplet+ a three-dimensional grid along each triplet of normal modes is computed.
41
42The number of grid points along each normal mode, or pair of modes can be defined
43by specifying:
44
45\begin{verbatim}
46    ngrid <integer default 16>
47\end{verbatim}
48
49This VSCF module by default calculates the ground state (v=0), but can also calculate excited
50states (such as v=1). The number of excited states calculated is defined by specifying:
51
52\begin{verbatim}
53    iexcite <integer default 1>
54\end{verbatim}
55
56With \verb+iexcite=1+ the fundamental frequencies are calculated.
57With \verb+iexcite=2+ the first overtones are calculated.
58With \verb+iexcite=3+ the second overtones are calculated.
59
60In certain cases the pair coupling potentials can become larger than those for a single
61normal mode. In this case the pair potentials need to be scaled down. The scaling factor
62used can be defined by specifying:
63
64\begin{verbatim}
65    vcfct <real default 1.0>
66\end{verbatim}
67
68
69