53.1 Polynomial representations (POLY)


The POLY program allows for the transformation of the potential energy surface and property surfaces from a grid representation to a polynomial representation. Once a polynomial representation has been chosen, the corresponding VSCF, VCI or VMP2 programs need to be selected (see below). The POLY program is fully parallelized in terms of the MPPX scheme. An example for the use of the POLY program can be found in chapter 55.1.3.

The following options are available:

The keyword NDIM=$n$ terminates the transformation after the $n$D terms within the $n$-mode expansion of the surfaces. The default is set to 3. The transformation of the 4D terms can be very time consuming.
Term after which the $n$-body expansions of the dipole surfaces are truncated. The default is set to 3. Note that NDIMDIP has to be lower or equal than NDIM.
Term after which the $n$-body expansions of the polarizability tensor surfaces are truncated. The default is 0. NDIMPOL has to be lower or equal than NDIM and must be smaller than 4.
Once the value of the NGRID=$n$ keyword for controlling the number of grid points has been changed in the SURF program, this information will be passed to the POLY program automatically.
AUTOPOL=1 (default) determines the optimal parameters for PMAX and PSUM in an iterative procedure. In combination with AUTOPOL=1 the PMAX and PSUM keywords specify the maximum values within the automated determination of these parameters.
This keyword controls the order of the polynomial to be determined in the fitting procedure. The default is set to 8. The definition of this keyword changes in combination with the AUTOPOL keyword (see above).
In multidimensional fitting the maximum order of the polynomial is not given by PMAX times the dimension, but rather by PSUM. The default is set to 24. This reduces the computational cost for fitting tremendously. However, for accurate calculations PMAX=12 and PSUM=30 are recommended. The definition of this keyword changes in combination with AUTOPOL keyword (see above).
Once polynomials have been generated, the expansion of the potential can be restricted to the harmonic approximation or a semi-quartic force field, i.e. TYPE=HARM or TYPE==QFF.
In case that dipole surfaces have been computed, they need to be transformed to a polynomial representation as well. This can be accomplished by DIPOLE=1 and will be set by default, if dipole surfaces are available. DIPOLE=0 excludes existing dipole surfaces from the transformation.
Once set to one this keyword switches the transformation of the polarizability tensor surfaces on. The default is POLAR=0.
The Watson correction term, i.e. VAM=1, is absorbed in the potential by default. Once the polynomials to be generated should exclude this correction, this needs to be specified by VAM=0. Any other values, i.e. 2 or 3, will be ignored.
In order to delete troublesome 2D, 3D or 4D surfaces from the multi-mode expansion of the potential, the DELAUTO keyword sets all coefficients of the polynomial expansion of these surfaces to zero. The threshold passed to the DELAUTO keyword corresponds to the $\chi^2$ value of the least squares fitting procedure. It acts on both, the energy and the dipole surfaces. The default is set to 9.d99, i.e. all surfaces with $\chi^2$ values below this threshold will not be affected.
By default, SOLVER=1, an LU decomposition will be used within the fitting procedure. Alternatively, a Cholesky decomposition is available, which sometimes leads to more stable solutions (SOLVER=2).
For the calculation of vibrationally averaged rotational constants $\mu$-tensor surfaces will be generated and transformed to polynomials up to 2nd order (default). The order can be changed by the integer passed to the NVARC keyword. This transformation can also be extended within the VSCF program.
Once vibrational-rotational coupling surfaces have been computed in the SURF program, these couplings can be considered (VRC=1) or excluded (VRC=0, default) in the POLY program.
The coefficients of the polynomial representation can be printed. In order to identify quartic potentials, it is recommended to use SHOW=1. Higher values will lead to a very long output file.
This keyword controls the record, from which the potential shall be read. The default is 5600.2. This keyword usually is only needed within the calculation of vibronic spectra.
Specification of the record, where to dump the polynomials. The default is 5750.2. This keyword usually is only needed within the calculation of vibronic spectra.
INFO=1 provides a list of the values of all relevant program parameters (options).

molpro@molpro.net 2019-06-17