45.1 Options

The following options are available:

VERSION=value

The default (i.e. VERSION=1) is a configuration selective VCI implementation, which is the fastest VCI code available in MOLPRO. The configuration selection is based on the vibrational ground state, while the final VCI matrix is state-specific. This approximation reduces the computational effort within the screening. VERSION=2 is a state-specific configuration selective VCI implementation, which usually is more reliable - but also more expensive - than VERSION=1. However, the configuration selection still introduces some inaccuracies which may not be desirable for very accurate calculations. A small correction to the state-specific configuration selection VCI may be invoked by choosing VERSION=3. This may increase the CPU-time by a factor of two. All configuration selective VCI programs have been parallelized in a fashion, that the individual columns of the VCI matrices are send to different processors. A conventional VCI calculation can be performed by choosing VERSION=4. This version yields the most accurate results. However, in this case the VCI matrix can be huge and usually these calculations are limited either by the CPU demands for diagonalizing the VCI matrix or by the corresponding memory requests.

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.

BSIZE=value

The size of the screening blocks within the configuration selection algorithm is measured in terms of the number of single and double excitations, i.e. BSIZE=1.0. The default is BSIZE=1.5 (for details see the reference given above).

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. 20) 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.

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.

INFO=value

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