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Subsections
8.49 vibrot
The program VIBROT is used to compute a vibrationrotation
spectrum for a diatomic molecule, using as input a potential
computed over a grid. The grid should be dense around equilibrium (recommended
spacing 0.05 au) and should extend to large distance (say 50 au) if
dissociation energies are computed.
The potential is fitted to an analytical form using cubic splines. The
rovibrational SchrÃ¶dinger equation is then solved numerically
(using Numerov's method) for one vibrational state at a time and for a
number of rotational quantum numbers as specified by input. The
corresponding wave functions are stored on file
VIBWVS for later use. The rovibrational energies
are analyzed in terms of spectroscopic constants. Weakly bound potentials can be
scaled for better numerical precision.
The program can also be fed with property functions, such as a dipole moment
curve. Matrix elements over the rovib wave functions for the property in
question are then computed. These results can be used to compute IR
intensities and vibrational averages of different properties.
VIBROT can also be used to compute transition properties between
different electronic states. The program is then run twice to produce two files
of wave functions. These files are used as input in a third run, which will
then compute transition matrices for input properties. The main use is to
compute transition moments, oscillator strengths, and lifetimes for rovib
levels of electronically excited states. The asymptotic energy difference
between the two electronic states must be provided using the ASYMptotic
keyword.
8.49.1 Dependencies
The VIBROT is freestanding and does not depend on any
other program.
8.49.2 Files
The calculation of vibrational wave functions and spectroscopic
constants uses no input files (except for the standard input).
The calculation of transition properties uses
VIBWVS files from two preceding
VIBROT runs, redefined as
VIBWVS1 and
VIBWVS2.
VIBROT generates the file
VIBWVS with vibrational wave functions for each v and J quantum
number, when run in the wave function mode. If requested VIBROT can
also produce files VIBPLT with the fitted potential and property
functions for later plotting.
8.49.3 Input
This section describes the input to the VIBROT program in the
MOLCAS program system. The program name is
&VIBROT
8.49.3.1 Keywords
The first keyword to
VIBROT is an indicator for the type of calculation
that is to be performed. Two possibilities exist:
Keyword  Meaning

ROVIbrational spectrum  VIBROT will perform a vibrot analysis and compute
spectroscopic constants.

TRANsition moments  VIBROT will compute transition moment integrals
using results from two previous calculations of the vibrot wave
functions. In this case the keyword Observable should be
included, and it will be interpreted as the transition dipole moment.

Note that only one of the above keywords can be used in a single
calculation. If none is given the program will only process the input
section.
After this first keyword follows a set of keywords, which are used to
specify the run. Most of them are optional.
The compulsory keywords are:
Keyword  Meaning

ATOMs  Gives the mass of the two atoms. Write mass number (an integer) and the
chemical symbol Xx, in this order, for each of the two atoms in free format. If
the mass numbers is zero for any atom, the mass of the most abundant isotope
will be used. All isotope masses are stored in the program. You may introduce
your own masses by giving a negative integer value to the mass number (one of
them or both). The masses (in unified atomic mass units, or Da) are then read
on the next (or next two) entry(ies). The isotopes of hydrogen can be given as
H, D, or T.

POTEntial  Gives the potential as an arbitrary number of lines. Each line
contains a bond distance (in au) and an energy value (in au). A plot file of the
potential is generated if the keyword
Plot is added after the last energy input. One more entry should then follow
with three numbers
specifying the start and end value for the internuclear distance and
the distance between adjacent plot points. This input must only be
given together with the keyword RoVibrational spectrum.

In addition you may want to specify some of the following optional
input:
Keyword  Meaning

TITLe  One single title line

GRID  The next entries give the number of grid points used in the numerical
solution of the radial SchrÃ¶dinger equation. The default value is
199. The maximum value that can be used is 4999.

RANGe  The next entry contains two distances Rmin and Rmax (in au) specifying
the range in which the vibrational wave functions will be computed.
The default values are 1.0 and 5.0 au. Note that these values most
often have to be given as input since they vary considerably from one
case to another. If the range specified is too small, the program will
give a message informing the user that the vibrational wave function
is large outside the integration range.

VIBRational  The next entry specifies the number of vibrational quanta for which the
wave functions and energies are computed. Default value is 3.

ROTAtional  The next entry specifies the range of rotational quantum numbers.
Default values are 0 to 5. If the orbital angular momentum quantum
number () is non zero, the lower value will be adjusted to
if the start value given in input is smaller than
.

ORBItal  The next entry specifies the value of the orbital angular momentum
(0,1,2, etc). Default value is zero.

SCALe  This keyword is used to scale the potential, such that the
binding energy is 0.1 au. This leads to better precision in the numerical
procedure and is strongly advised for weakly bound potentials.

NOSPectroscopic  Only the wave function analysis will be carried out but not the
calculation of spectroscopic constants.

OBSErvable  This keyword indicates the start of input for radial functions of observables
other than the energy, for example the dipole moment function. The next line
gives a title for this observable. An arbitrary number of input lines follows.
Each line contains a distance and the corresponding value for the observable.
As for the potential, this input can also end with the keyword Plot,
to indicate that a file of the function for later plotting is to be constructed.
The next line then contains the minimum and maximum Rvalues and the
distance between adjacent points. When this input is given with the top keyword
RoVibrational spectrum the program will compute matrix elements for
vibrational wave functions of the current electronic state. Transition moment
integrals are instead obtained when the top keyword is Transition
moments. In the latter case the calculation becomes rather meaningless if
this input is not provided. The program will then only compute the overlap
integrals between the vibrational wave functions of the two states.
The keyword Observable can be repeated up to ten times in a
single run. All observables should be given in atomic units.

TEMPerature  The next entry gives the temperature (in K) at which the vibrational
averaging of observables will be computed. The default is 300 K.

STEP  The next entry gives the starting value for the energy step used in
the bracketing of the eigenvalues. The default value is 0.004 au
(88 cm^{1}). This value must be smaller than the
zeropoint vibrational energy of the molecule.

ASYMptotic  The next entries specifies the asymptotic energy difference between
two potential curves in a calculation of transition matrix elements.
The default value is zero atomic units.

ALLRotational  By default, when the Transition moments keyword is given, only the
transitions between the lowest rotational level in each vibrational state are
computed. The keyword AllRotational specifies that the transitions
between all the rotational levels are to be included. Note that this may result
in a very large output file.

PRWF  Requests the vibrational wave functions to be printed in the output file.

&VIBROT
RoVibrational spectrum
Title = VibRot spectrum for FeNi
Atoms = 0 Fe 0 Ni
Potential
1.0 0.516768
1.1 0.554562
[...]
Plot = 1.0 10.0 0.1
Grid = 150
Range = 1.0 10.0
Vibrations = 10
Rotations = 2 10
Orbital = 2
Observable
Dipole Moment
1.0 0.102354
1.1 0.112898
[...]
Plot = 1.0 10.0 0.1
Comments: The vibrationalrotation spectrum for FeNi
will be computed using the potential curve given in input. The 10
lowest vibrational levels will be obtained and for each level the
rotational states in the range J=2 to 10. The vibrot matrix elements
of the dipole function will also be computed. A plot file of the
potential and the dipole function will be generated. The masses for
the most abundant isotopes of Fe and Ni will be selected.
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