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vibration_correlation_programs [2021/07/28 13:35] rauhuterfort [Rovibrational calculations] 
vibration_correlation_programs [2022/01/14 15:04] (current) rauhutdennisdinu [Rovibrational calculations] 
* **''CIMAX''=//value//** ''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=12'' for 3D potentials and ''CIMAX=15'' for 4D potentials.  * **''CIMAX''=//value//** ''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=12'' for 3D potentials and ''CIMAX=15'' for 4D potentials. 
* **''CITYPE''=//n//** ''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) for 3D potentials and ''CITYPE=5'' for 4D potentials, which appears to be a fair compromise between accuracy and computational speed. The maximum excitation level is currently limited to ''CITYPE=9''.  * **''CITYPE''=//n//** ''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) for 3D potentials and ''CITYPE=5'' for 4D potentials, which appears to be a fair compromise between accuracy and computational speed. The maximum excitation level is currently limited to ''CITYPE=9''. 
 * **''CLASSES''=//n//** CLASSES=1 allows for the rigorous use of the SlaterCondon rules by employing configuration classes in configuration selection and VCI matrix setup. 
* **''CLCTYPE''=//n//** (=2 Default) All VCI programs are based on a real configuration basis. If the state of interest is specified by the $l$ quantum number (symmetric top or linear molecules) an appropriate start vector for the iterative eigenvalue solver will be generated automatically. The option ''CLCTYPE'' allows to use different start vectors.  * **''CLCTYPE''=//n//** (=2 Default) All VCI programs are based on a real configuration basis. If the state of interest is specified by the $l$ quantum number (symmetric top or linear molecules) an appropriate start vector for the iterative eigenvalue solver will be generated automatically. The option ''CLCTYPE'' allows to use different start vectors. 
* **''COMBI''=//n//** 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''.  * **''COMBI''=//n//** 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''. 
* **''SADDLE''=//n//** 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 the ''VCI'' program by using ''SADDLE=1''. Currently, the ''VCI'' program can only handle symmetrical doubleminimum potentials.  * **''SADDLE''=//n//** 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 the ''VCI'' program by using ''SADDLE=1''. Currently, the ''VCI'' program can only handle symmetrical doubleminimum potentials. 
* **''SELSCHEME''=//n//** By default ''SELSCHEME''=1, configurationis will be selected by a perturbative criterion. Alternative one may use a criterion based on 2$\times$2 VCI matrices (''SELSCHEME''=2). Usually the differences are extremely small and the matrix based criterion is slightly more timeconsuming.  * **''SELSCHEME''=//n//** By default ''SELSCHEME''=1, configurationis will be selected by a perturbative criterion. Alternative one may use a criterion based on 2$\times$2 VCI matrices (''SELSCHEME''=2). Usually the differences are extremely small and the matrix based criterion is slightly more timeconsuming. 
 * **''SKIPDIAG''=//n//** If SKIPDIAG=ITER with ITER>1 is set, the configuration selection based on a VMP2like wavefunction is used after the ITERth iteration step, as long as the energy difference between the energy eigenvalues of the last two iteration steps is less than THRDIAG (see below). SKIPDIAG=2 usually leads to appropriate results. 
 * **''SKIPSAVE''=//n//** In cases of small deviations with respect to the norm of the VMP2like wavefunction, SKIPSAVE=1 may be used to further reduce the computational cost for configuration selection. Note, that renormalization is not possible anymore in this case. The default is SKIPSAVE=0 (disabled). 
 * **''SUBSPACE''=//n//** SUBSPACE=1 enables the use of prediagonalization of physically meaningful subspaces. 
* **''SYMBASIS''=//n//** The assignment of symmetry labels is realized by a projection operator. By default, the basis functions used are automatically determined. ''SYMBASIS=1'' switches to alternative basis functions.  * **''SYMBASIS''=//n//** The assignment of symmetry labels is realized by a projection operator. By default, the basis functions used are automatically determined. ''SYMBASIS=1'' switches to alternative basis functions. 
* **''THERMO''=//n//** ''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''.  * **''THERMO''=//n//** ''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''. 
* **''THRCF''=//value//** ''THRCF'' is the threshold for selecting individual configurations. The default is given by ''THRCF''=$5\cdot 10^{10}$.  * **''THRCF''=//value//** ''THRCF'' is the threshold for selecting individual configurations. The default is given by ''THRCF''=$5\cdot 10^{10}$. 
 * **''THRDIAG''=//value//** The default is THRDIAG=1.0d0 (wavenumbers). THRDIAG gives the energy difference of the energy eigenvalues between two consecutive iterations. The VMP2like wavefunction is employed for configuration selection if the respective energy difference is smaller than THRDIAG, i.e. VCI matrix diagonalizations/eigenvector determinations are omitted. 
 * **''THRECORR''=//value//** Convergence criterion in the case the VMP2like wavefunction is used for configuration selection. If the sum of energy corrections given by the configurations selected in a specific iteration step is smaller than THRECORR, the final VCI matrix is setup and the eigenvector is determined. The default is THRECORR=1.0d0 (wavenumbers). 
* **''THREX''=//value//** This thresholds controls the exclusion of selected configurations within the perturbative configuration selection criterion. The default is ''THREX=5.d4''.  * **''THREX''=//value//** This thresholds controls the exclusion of selected configurations within the perturbative configuration selection criterion. The default is ''THREX=5.d4''. 
* **''THRSEL''=//value//** ''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''=0.02 cm$^{1}$.  * **''THRSEL''=//value//** ''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''=0.02 cm$^{1}$. 
* **''RAMAN_FAC(n)''=//value//** Set the prefactors for the isotropic and anisotropic Raman transition moments for the calculation of Raman intensities manually. $n=0$ will set the value for $R_0$, $n=2$ the one for $R_2$.  * **''RAMAN_FAC(n)''=//value//** Set the prefactors for the isotropic and anisotropic Raman transition moments for the calculation of Raman intensities manually. $n=0$ will set the value for $R_0$, $n=2$ the one for $R_2$. 
* **''RAMAN_LFREQ''=//value//** (=680 Default, in nm) Raman exciting radiation (laser) frequency.  * **''RAMAN_LFREQ''=//value//** (=680 Default, in nm) Raman exciting radiation (laser) frequency. 
 * **''PFIT''=//value//** (=0 Default) Fitting of spectroscopic parameters for asymmetric tops, using Watson's reduced operator. ''PFIT=1'' activates the fitting procedure if ''JMAX>0''. Intensities are not needed for the fitting. Currently, the spectroscopic parameters are fitted only with respect to the vibrational ground state. By setting ''PFIT=2'' or higher, additional printout is provided. 
 * **''PFITRED''=//A/S//** (=A Default) Select between A and Sreduction. 
 * **''PFITDAT''=//string//** File name for dumping the term energies for the fitted spectroscopic parameters. 
 
 