An incomplete fourth-order Taylor series containing linear, quadratic, bilinear, cubic and quartic terms is used to fit to each element.
The group theoretical arguments detailed here are used to determine whether a given parameter of the Taylor expansions is zero by symmetry.
The output of the LMFIT program consists of a single ascii file
containing the values of the fitted parameters in a human-readable
format. For an input file named file.inp
, the output file
is named file.lmi
.
or
lmfit input
where input (or input.inp) denotes the input file. The input format uses, like all the Quantics package input files, keywords that are for the most part free format and case insensitive. See Quantics input file structure
lmfit input.inp
for further information on the general use of keywords, noting that there are no sections in the VCHAM input files. The input file ends with the keyword end-input
Fit Specification | ||
---|---|---|
Keyword | Description | |
infofile = A | The database written by LMTRANS to be used is called A | |
fit_select = m1,m2,... | The optimisation will be of terms defined with respect to the modes m1,m2,... | |
dipole_fit | Flags that the parameters entering into the expansion of the dipole matrix are to be optimised | |
polar_fit | Flags that the parameters entering into the expansion of the polarisability tensor are to be optimised |
The symmetries of the normal modes are read from the specified LMTRANS database file.
The point group is specified using the syntax
point_group = G
where G is the name of the point group (Cs, C2v, etc). Only Abelian point groups are supported. If the point group of the molecule is non-Abelian, the highest Abelian subgroup should be used.
The symmetries of the states and vectors pointing along the x-, y-, and x-axes have to be given in the input as follows.
(1) State symmetries
The symmetries of the states is specified between the lines
state_symmetry
and
end-state_symmetry
For example, for the C2v point group
state_symmetry
1 A1
2 B1
.
.
.
end-state_symmetry
would specify that the state 1 has A1 symmetry, state two has B1 symmetry, and so on.
(2) Axis symmetries
The symmetries of the axes is specified between the lines
axis_symmetry
and
end-axis_symmetry
For example, for the C2v point group
state_symmetry
x B1
y B2
z A1
end-state_symmetry
would specify that a vector lying along the x-axis generates the B1 representation, and so on.