52.1 Options

The following options are available:
The keyword NDIM=$n$ terminates the expansion of the PES after the $n$-body term. Currently, at most 4-body terms can be included, but the default is set to 3. Please note, when you use NDIM=4 as a keyword for the SURF program, you need to pass this information to the VSCF and VCI programs also. Otherwise these programs will neglect the 4-body terms.
Based on a coarse grid of ab initio points a fine grid will be generated from automated interpolation techniques. The keyword NGRID=$n$ determines the number of equidistant grid points in one dimension. NGRID=$n$ has to be an even number. The default is currently set to 16. Note that the number of grid points also controls the extension of the $n$-dimensional potential energy surfaces (see keyword SCALE) and thus influences many internal thresholds which are optimized to the default value of NGRID. The number of grid points also determines the number of basis functions in the grid-based VSCF program. At present the maximum grid size is 36.
Grid points 14 16 18 20
Surface extension 4.30 4.69 5.05 5.39
The SURF program reads the energy of electronic structure calculations from the internal MOLPRO variables, e.g. ENERGY, EMP2, $\dots$. The internal variable is specified by the keyword VAR1D. Within the example shown above, VAR1D=ENERGY would read the CCSD energy, while VAR1D=EMP2 would read the MP2 energy, which is a byproduct of the CCSD calculation. The default for the VAR1D keyword is the internal variable ENERGY.
Symmetry within electronic structure calculations can be exploited by the keyword SYM=Auto. Usually this leads to significant time savings. By default this symmetry recognition is switched off as certain calculations may cause some trouble (e.g. local correlation methods). Symmetry in electronic structure calculations may not be mistaken by the symmetry of the mode-coupling terms (see keyword MPG).
Symmetry of the normal modes is recognized by the program automatically. Only Abelian point groups can be handled at the moment. Symmetry of the modes will be determined even if the NOSYM keyword is used in the electronic structure calculations. In certain cases numerical noise can be very high and thus prohibits a correct determination of the symmetry labels. Symmetry can be switched off by using MPG=1.
The extension of the potential energy surfaces is determined from Gauss-Hermite quadrature points. Using a fine grid NGRID=16 the surface stretches out to the NGRID/2$^{th}$ Gauss-Hermite point, i.e. 4.69, in each direction (see keyword NGRID). As these values are fairly large within the calculation of fundamental modes, a scaling factor, SCALE=$f$, has been introduced. A default scaling of 0.75 is used. Increasing the size of the surfaces usually requires the calculation of further ab initio points as the surface interpolation is more stable for surfaces of limited size.
The iterative algorithm for generating potential energy surfaces is based on a successive increase of interpolation points. The iterations are terminated once the interpolation of two subsequent iteration steps became stable. The convergence threshold can be changed by the keyword THRFIT=$f$. There is currently just one control variable for the different 1D, 2D, 3D, and 4D iterations. The 4 thresholds are different but depend on each other. Consequently, changing the default value (THRFIT=4.0d-2) will change all thresholds simultaneously which keeps the calculation balanced.
The maximum order of the polynomials used for fitting within the iterative interpolation scheme can be controlled by the keywords FIT1D, FIT2D, FIT3D, FIT4D. The default is given by 8. However in certain cases higher values may be necessary, but require an appropriate number of coarse grid points, which can be controlled by MIN1D etc.
The minimum number of coarse grid points can be controlled by the keywords MIN1D, MIN2D, MIN3D, MIN4D. These 4 keywords determine the minimum number of ab initio calculations in one dimension for each 1D, 2D, 3D and 4D surface. The defaults are currently MIN1D=4, MIN2D=4, MIN3D=4, MIN4D=4. Presently, values larger than 24 are not supported.
The maximum number of coarse grid points can be controlled by the keywords MAX1D, MAX2D, MAX3D, MAX4D. These 4 keywords determine the maximum number of ab initio calculations in one dimension for each 1D, 2D, 3D and 4D surface. The defaults are currently MAX1D=24, MAX2D=16, MAX3D=10, MAX4D=8. Presently, values larger than 24 are not supported.
Outer regions of the potential energy surfaces may be determined by extrapolation rather than interpolation schemes. By default extrapolation is switched off, i.e. Ext12D=1.0 and Ext34D=1.0. However, an extrapolation of 10% in case of the 1D and 2D contributions to the potential (Ext12D=0.9) and of 20% in case of the 3D and 4D terms (Ext34D=0.8) may be useful as it usually stabilizes the fitting procedure.
As the number of 3D and 4D surfaces can increase very rapidly, there exists the possibility to neglect unimportant 3D and 4D surfaces by the keywords SKIP3D and SKIP4D. The criterion for the prescreening of the 3D surfaces is based on the 2D terms and likewise for the 4D terms the 3D surfaces are used. The neglect of 3D surfaces automatically leads to the neglect of 4D surfaces, as the latter depend on the previous ones. By default prescreening is switched on, but can be switched off by SKIP3D=0.0 and SKIP4D=0.0.
Once the keyword VRC=1 is provided, the SURF program will also compute the vibrational-rotational coupling surfaces and thus increases the number of degrees of freedom to 3N-3. Vibrational-rotational coupling surfaces can only be used within the PESTRANS program (see below), but will be neglected in any VSCF or VCI calculations.
After calculating a number of grid points within the iterative interpolation scheme the convergence of the individual surfaces will be checked and, if provided by the keyword DUMP, dumped to disk. This leads typically to 3-5 iterations and thus the same number of restart points within the calculation of the 1D, 2D, ... surfaces. As the number of 3D and 4D terms can be very large this is not sufficient in these cases. Therefore, the lists of 3D and 4D terms is cut into batches which will be processed subsequently. BATCH3D and BATCH4D control the number of 3D and 4D surfaces within each batch. By default BATCH3D is set to 30 times the number of processors and BATCH4D to 10 times the number of processors. Accordingly the number of restart points is increased. Smaller values for BATCH3D and BATCH4D, e.g. BATCH3D=20, increase the number of restart points on cost of the efficiency of the parallelization.
Standard SURF calculations expect the reference structure to be a (local) minimum on the PES, i.e. SADDLE=0 (default). Alternatively, one may start the PES generation from a transition state, which is recommended for the calculation of double-minimum potentials. This situation is not recognized automatically and thus requires the keyword SADDLE=1.
TYPE=QFF calls a macro, which modifies the parameters of the SURF program in order to compute a quartic force field in the most efficient manner. This implies a reduction of the size of the coupling surfaces and a limitation of the maximum number of points for the $n$D-terms. It should be used for VPT2 calculations. TYPE=ZPVE calls a macro, which changes the defaults for several parameters of the SURF, VSCF and VCI programs. It is meant for the quick and efficient calculation of zero point vibrational energies on cost of some accuracy. For example, the expansion of the potential will be truncated after the 2D terms. As a consequence the output of course is reduced to the presentation of the vibrational ground state only. TYPE=FULL (default) performs a standard calculation as needed for VSCF or VCI calculations.
Once the MRCC program of M. Kallay is used for determining individual grid points, the option USEMRCC=1 needs to be set, which is needed to ensure proper communication between MOLPRO and MRCC. Default: USEMRCC=0.
For large molecules or in the case of modelling the 3D and 4D terms, the .log-file may become huge. First of all the .log-file can be directed to scratch within the electronic structure program, i.e. logfile, scratch. The option DELLOG=1 always truncates the .log-file in a way that it contains only the very last energy calculation. Default: DELLOG=0.
INFO=1 provides a list of the values of all relevant program parameters (options). Default: INFO=0.
PLOT=n plots all nD surfaces and a corresponding GNUPLOT script in a separate subdirectory (plots1) in the home-directory in order to allow for visualization of the computed nD surfaces. E.g. the command "gnuplot plotV1D.gnu" in the plots1 directory produces .eps files for all 1D surfaces. Default: PLOT=0.

The following example shows the input of a calculation which computes energy and dipole surfaces at the MP2/cc-pVTZ level and subsequently determines the anharmonic frequencies at the VSCF and VCI levels. Hartree-Fock calculations will not be restarted and the .log-file is directed to the scratch directory as defined by the $TMPDIR variable.

O          0.0675762564        0.0000000000       -1.3259214590
H         -0.4362118830       -0.7612267436       -1.7014971211
H         -0.4362118830        0.7612267436       -1.7014971211





molpro@molpro.net 2019-06-16