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50.1 Options

The following options are available:

TYPE=value

VCI solutions can be obtained using a potential in grid representation, i.e. TYPE=GRID, 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.

VERSION=value

Both, the grid-based and the polynomial-based versions of the VCI programs offer 4 different kinds of VCI implementations: VERSION=1 is the fastest program. It works state-specific and configuration selective and makes use of a simultaneous exclusion and internal contraction scheme (see the reference given above). VERSION=2 is identical with VERSION=1 but switches off the internal contractions. VERSION=3 is the most accurate configuration selective VCI program and does neither use the internal contraction scheme nor the simultaneous exclusion. VERSION=4 is a conventional VCI program without any of the aforementioned approximations. It is thus computationally extremely demanding.

CITYPE=value

CTYPE 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=value

LEVEX determines the level of excitation within one mode, i.e. $0\rightarrow 1$, $0\rightarrow 2$, $0\rightarrow 3$, ... The default is LEVEX=4, which was found to be sufficient for many applications.

CIMAX=value

CIMAX is the maximum excitation level corresponding to CITYPE and LEVEX. In principle, a triple configuration $(4,4,4)$ would contribute to the VCI space. However, CIMAX=7 restricts this to $(4,3,0)$, $(3,3,1)$, $(3,2,2), ...$. The default is CIMAX=8, which needs to be extended for certain applications.

CITHR=value

CITHR controls the threshold within the configuration selection scheme (see VERSION=1-3). The default is 0.99995.

PMP=value

Vibrational angular momentum terms (Coriolis coupling),
i.e. $\frac{1}{2} \sum_{\alpha\beta} \hat{\pi}_\alpha\mu_{\alpha\beta} \hat{\pi}_\beta$, and the Watson correction term are by default switched off. PMP=1 adds the Watson correction term (see eq. 69) as a pseudo-potential like contribution to the fine grid of the potential. PMP=2 allows for the calculation of the integrals of the PMP operator using the approximation that the $\mu$ tensor is given as the inverse of the moment of inertia tensor at equilibrium geometry. The PMP=2 option includes diagonal contributions in the VCI matrix only. This is significantly faster than calculating the contributions for all matrix elements (which corresponds to PMP=3 and usually introduces only small deviations. PMP=4 extends the constant $\mu$-tensor (0D) by 1D terms. PMP=5 introduces 2D corrections to the $\mu$-tensor. PMP=4 and PMP=5 make use of a prescreening technique in which the convergence of the PMP operator will be checked for each VCI matrix element. PMP=8 corresponds to PMP=5 without prescreening. Note that the 1D and 2D corrections increase the computational cost considerably and are only available for non-linear molecules.

COMBI=value

By default the VCI program calculates the fundamental modes of the molecule only. However, choosing COMBI=1 allows for the calculation of the first vibrational overtones and $n\times(n-1)/2$ combination bands consisting of two modes in the first vibrational level.

THERMO=value

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 ist 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.

ROTJ=value

Rovibrational levels can be computed in an approximative fashion only (this does not work in combination with the COMBI option). Once the VCI wave function has been determined, the rotational constants will be computed from the vibrationally averaged structures for each vibrational level. This allows for a rough estimate and very fast calculation of the rovibrational levels. ROTJ=$n$ determines the value of $J$. A negative number for $J$ results in a calculation of all rovibrational levels from $J=1$ up to the specified $J$ value.

DIPOLE=value

DIPOLE=1 allows for the calculation of infrared intensities. Calculation of infrared intensities requires the calculation of dipole surfaces within the SURF program. By default intensities will not be computed.

NDIM=value

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.

MPG=value

Symmetry of the molecule will be recognized automatically within the VCI calculations. MPG=1 switches symmetry off.

NBAS=value

The number of basis functions (distributed Gaussians) to be used for obtaining the VCI solutions can be controlled 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.

DIAG=value

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.

INFO=value

INFO=1 provides a list of the values of all relevant program parameters (options).

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