55.1.1 Options

The following options are available:
VCI solutions can be obtained using a potential in grid representation, i.e. TYPE=GRID (default), or in a polynomial representation, TYPE=POLY. In the latter case the POLY program needs to be called prior to the VSCF and VCI programs in order to transform the potential.
By default, i.e. SADDLE=0, the VCI program assumes, that the reference point of the potential belongs to a local minimum. Once the PES calculation has been started from a transition state, this information must be provided to VCI program by using SADDLE=1. Currently, the VCI program can only handle symmetrical double-minimum potentials.
Both, the grid-based and the polynomial-based versions of the VCI programs offer 2 different kinds of VCI implementations:
VERSION=3 (which is the default) is a configuration selective and most efficient VCI program. VERSION=4 is a conventional VCI program without configuration selection. It is thus computationally extremely demanding.
CITYPE defines the maximum number of simultaneous excitations, i.e. Singles, Doubles, Triples, ... and thus determines the kind of calculations, i.e. VCISD, VCISDT, ... The default is CITYPE=4 (VCISDTQ), which appears to be a fair compromise between accuracy and computational speed. The maximum excitation level is currently limited to CITYPE=6.
LEVEX determines the level of excitation within one mode, i.e. $0\rightarrow 1$, $0\rightarrow 2$, $0\rightarrow 3$, ... The default is LEVEX=5, which was found to be sufficient for many applications.
CIMAX is the maximum excitation level corresponding to CITYPE and LEVEX. In principle, a triple configuration $(1^42^43^4)$ would contribute to the VCI space. However, CIMAX=7 restricts this to $(1^42^3)$, $(1^32^33^1)$, $(1^32^23^2), ...$. The default is CIMAX=9, which needs to be extended for certain applications.
THRSEL controls the determination of the iterative configuration selection scheme. By default the wavefunction is considered to be converged once energy changes drop below THRSEL=0.05 cm$^{-1}$.
THRCF is the threshold for selecting individual configurations. The default is given by THRCF=$10^{-9}$.
Within the evaluation of the VCI integrals contraction schemes are used to reduce the computational effort. In the polynomial based VCI program values 0, 1 and 2 are supported, while in the grid based version only the options 0 and 1 exist. Memory demands and CPU speed-ups increase with increasing values. The default is set to 1. On machines with limited memory a value of 0 is recommended for this keyword.
VAM=0: switches off all vibrational angular momentum terms and the Watson correction term.
VAM=1: adds the Watson correction term (see eq. 65) as a pseudo-potential like contribution to the fine grid of the potential.
VAM=2: (default) the 0D terms of the vibrational angular momentum terms, i.e. $\frac{1}{2} \sum_{\alpha\beta} \hat{\pi}_\alpha\mu_{\alpha\beta} \hat{\pi}_\beta$, and the Watson correction term are included. The VAM-terms will be added to the diagonal elements of the VCI-matrix only. This approximations works rather well for many applications.
VAM=3: again, the $\mu$ tensor is given as the inverse of the moment of inertia tensor at equilibrium geometry, but is added to all elements of the VCI matrix.
VAM=4: extends the constant $\mu$-tensor (0D) by 1D terms and is added to all elements of the VCI matrix. A prescreening technique is used for the 1D terms, in which the convergence of the VAM operator will be checked for each VCI matrix element.
VAM=5: includes 0D, 1D and 2D terms of the $\mu$-tensor, which are added to all elements of the VCI matrix. Prescreening is used for the 1D and 2D terms.
VAM=6: includes 0D and 1D terms of the $\mu$-tensor without prescreening.
VAM=7: includes 0D, 1D and 2D terms of the $\mu$-tensor, which are added to all elements of the VCI matrix. Prescreening is used for the 2D terms only.
VAM=8: includes 0D, 1D and 2D terms of the $\mu$-tensor without any prescreening.
Note that the 1D and 2D corrections increase the computational cost considerably and are only available for non-linear molecules.
By default the VSCF program calculates the fundamental modes of the molecule only. However, choosing COMBI=$n$ allows for the calculation of the vibrational overtones and combination bands. The value of $n$ controls the excitation level, i.e. the number of states to be computed increases very rapidly for large values of $n$. Therefore, by default the upper limit is set to 5000 cm$^{-1}$, but this cutoff can be changed by the option UBOUND.
Once vibrational states have been defined with the VIBSTATE program (section 54.3), the VCI program can be forced to compute just these states by the option USERMODE=1. Note that the vibrational ground state will always be computed and needs not to be specified explicitly.
Once overtones and combination bands shall be computed, the upper energy limit is controlled by the keyword UBOND, i.e. states, for which the harmonic estimate is larger than $n$, will not be computed. the default is set to $n$=5000 cm$^{-1}$.
BASIS=1 (default) defines a mode-specific basis of distributed Gaussians, which is the recommended choice. However, for certain applications a mode-independent basis (BASIS=2) can be used as well. Very often this leads to worse results for torsional modes. BASIS=3 distributes the Gaussians in a way, that the overlap integral between two functions is always the same (controlled by THRBASOVLP. This guarantees that an increasing number of basis functions will always lead to an improvement.
Overlap between two Gaussian basis functions, once BASIS=3 has been chosen. The default is 0.75.
THERMO=1 allows for the improved calculation of thermodynamical quantities (compare the THERMO keyword in combination with a harmonic frequency calculation). However, the approach used here is an approximation: While the harmonic approximation is still retained in the equation for the partition functions, the actual values of the frequencies entering into these functions are the anharmonic values derived from the VCI calculation. Default: THERMO=0.
DIPOLE=1 (default) allows for the calculation of infrared intensities. Calculation of infrared intensities requires the calculation of dipole surfaces within the SURF program. By default the intensities will be computed on the basis of Hartree-Fock dipole surfaces.
POLAR=1 allows to compute Raman intensities in addition to infrared intensities, but of course requires polarizability tensor surfaces from the SURF program. By default Raman intensities are switched off.
The expansion of the potential in the VCI calculation can differ from the expansion in the SURF calculation. However, only values less or equal to the one used in the surface calculation can be used. Default: NDIM=3.
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 to NDIM.
Term after which the $n$-body expansions of the polarizability tensor surfaces are truncated. The default is set to 0. Note that NDIMPOL has to be lower or equal to NDIM and must be samller than 4.
By default the symmetry of the molecule will be recognized automatically within the VCI calculations. MPG=1 switches symmetry off.
The number of basis functions (distributed Gaussians) to be used for obtaining the VCI solutions can be controlled by NBAS=value. The number of basis functions must be identical to the number used in the VSCF program. The default is NBAS=20. This option is only active once a polynomial representation of the potential has been chosen, see the option TYPE=POLY and the POLY program.
This keyword specifies the reference for the definition of the configurations. By default, REFERENCE=0 the reference for all state-specific calculations is the vibrational ground-state configuration. This leads to a violation of the Brillouin condition, but often to also to faster convergence. REFERENCE=1 uses the VSCF configuration as reference for generating all excited configurations. This is the proper way of doing it, but usually requests higher excitation levels.
By default all VCI calculations will be done state-specifically, GSMODALS=0, i.e. the modals refer to the individual VSCF solutions. GSMODALS=1 uses the modals of the VSCF ground-state for all VCI calculations. This still requests an individual VCI calculation for each vibrational state (in contrast to just one VCI calculations from which all solutions will be retrieved) and thus the final VCI wave functions may not be strictly orthogonal to each other once the VCI space is incomplete.
In the polynomial configuration selective VCI program different diagonalization schemes can be used. DIAG=CON specifies a conventional non-iterative diagonalization as used in the grid-based versions. DIAG=JAC is the default and uses a Jacobi-Davidson scheme. DIAG=HJD denotes a disk-based Jacobi-Davidson algorithm.
In case of resonances or strongly mixed states in general (i.e. low VCI coefficients) a multi-state analysis can be performed, which prints major contributions of the VCI-vectors for all states in a certain window around the state of interest. Typically a window between 10 and 20% (i.e. ANALYZE=0.1 or ANALYZE=0.2) provides all the information needed. As this analyses requires a conventional diagonalization (see DIAG), the CPU time may increase significantly.
By default the expansion of the $\mu$-tensor for calculating the vibrationally averaged rotational constants is truncated after the 2nd order terms, i.e. NVARC=2. This may be altered by the NVARC keyword.
This option provides an extended output. PRINT=1 prints the vibrationally averaged rotational constants for all computed states and the associated vibration-rotation constants $\alpha$. PRINT=2 prints the effective 1D polynomials in case that the potential is represented in terms of polynomials, see the option TYPE=POLY and the POLY program. In addition the generalized VSCF property integrals, i.e. $\left < VSCF \left \vert q_i^r \right \vert VSCF \right >$ are printed. These integrals allow for the calculation of arbitrary vibrationally averaged properties once the property surfaces are available. Default: PRINT=0.
This card specifies the record from where to read the VSCF information. As the VSCF information usually is stored in the same record as the polynomials, it is usually defined in the POLY program. This option is only of importance within the calculation of vibronic spectra.
This keyword specifies the record where to dump the VCI information. This option is only of importance within the calculation of vibronic spectra.
If variable is set to FCON, important VCI information will be passed to the Franck-Condon calculation. Within Franck-Condon calculations this option has to be used.
INFO=1 provides a list of the values of all relevant program parameters (options).

molpro@molpro.net 2019-08-22