$VSCF group (optional, relevant to RUNTYP=VSCF) This group governs the computation of vibrational frequencies including anharmonic effects. Besides the keywords shown below, the input file must contain a $HESS input (and perhaps a $DIPDR input), to start with previously obtained harmonic vibrational information. The VSCF method requires only energies, so any energy type in GAMESS may be used, perhaps with fully numerical harmonic vibrational information. Energies are sampled along the directions of the harmonic normal modes, and usually along pairs of harmonic normal modes, after which the nuclear vibrational wavefunctions are obtained. The dipole on the grid points may be used to give improved IR intensities. The most accurate calculation computes the potential surface directly, on all grid points, but this involves many energy evaluations. An attractive alternative is the Quartic Force Field approximation of Yagi et al., which computes a fit to the derivatives up to fourth order by computing a specialized set of points, after which this fit is used to generate the full grid of points for the solver. Since there are a great many independent energy evaluations, no matter which type of surface is computed, the VSCF method allows for computations in subgroups (much like the FMO method). Thus any $GDDI input group will be read and acted upon, if found. Vibrational wavefunctions are obtained at an SCF-like level, termed VSCF, using product nuclear wavefunctions, along with an MP2-like correction to the vibrational energy, which is termed correlation corrected (cc-VSCF). In addition, vibrational energy levels based on second order degenerate pertubation theory (see VDPT) or a CI analog (see VCI) may be obtained. Most VSCF applications have been carried out with an electronic structure level of MP2 with triple zeta basis sets. This is thought to give accuracy to 50 wavenumbers for the larger fundamentals. Use of internal coordinates is known to give improved accuracy for lower frequencies, particularly in weakly bound clusters. Restarts involve the $VIBSCF input (which has different formats for each PETYP), and the READV keyword. Restarts are safest on the same machine, where normal mode phases are reproducible. References for the VSCF method, the QFF approximation, and the solvers are given in Chapter 4 of this manual, along with a number of sample applications. * * * * * The first input variables control the generation of the potential surface on which the nuclear vibrations occur: PETYP = DIRECT computes the full potential energy surface, according to NCOUP/NGRID. The total number of energy/dipole calculations for NCOUP=2 will be M*NGRID + (M*(M-1)/2)*NGRID*NGRID, where M is the number of normal modes. This is the default. = QFF the Quartic Force Field approximation to the potential surface is obtained. This is usually only slightly less accurate, but has a greatly reduced computational burden, namely 6*M + 12*M*(M-1)/2 energy/dipoles. INTCRD = flag setting the coordinate system used for the grids. Any internal coordinates to be used must be defined in $ZMAT, using 3N-6 simple, DLC, or natural internal coordinates. Of course, you must enter NZVAR in $CONTRL as well. The default is to use Cartesians (default .FALSE.) INTTYP = 0 default if INTCRD=.FALSE. (ignore this keyword) = 1 implies that the $ZMAT contains only stretches, bends, and torsions. It also selects an approximate transformation between Cartesian and internal coords. = 2 the other $ZMAT coordinates may be used, and the coordinate transformation will be iterated to convergence. (default if INTCRD=.TRUE.) NCOUP = the order of mode couplings included. = 1 computes 1-D grids along each harmonic mode = 2 adds additionally, 2-D grids along each pair of normal modes. (default=2) = 3 adds additionally, 3-D grids for mode triples, for PETYP=DIRECT only. NGRID = number of grid points to be used in solving for the anharmonic vibrational levels. In the case of PETYP=DIRECT, each of these grid points must be explicitly computed. For PETYP=QFF these grid points are obtained from a fitted quartic force field. Reasonable values are 8 or 16 for DIRECT, with 16 considered significantly more accurate. For PETYP=QFF, the generation of the solver grid is very fast, so use 16 always. (default=16) AMP = step size for PETYP=DIRECT displacements. The maximum distance along each mode is a function of its frequency, amplitude(i)=sqrt(2*(AMP+1/2)/freq(i)) so that AMP resembles a vibrational quantum number. The default goes far enough past the classical turning points of the fundamentals to capture the relevant part of the surface. (default = 7.0) STPSZ = step size for PETYP=QFF displacements. The step along each mode depends on the harmonic frequency, as well as this parameter, whose default is usually satisfactory (default=0.5) In case the user wants to control each normal mode with a separate parameter, arrays of values may be given, using the keywords AMPX(1)=xx,yy,... or STPSZX(1)=xx,yy,zz... IMODE = array of modes for which anharmonic effects will be computed. IMODE(1)=10,19 computes anharmonic energies and wavefunctions for modes 10 and 19, only. In the current implementation, pairs of modes cannot be coupled, so NCOUP is forced to 1 if this option is specified. This approximation is intended for larger molecules, where the whole VSCF calculation is prohibitive. * * * * * The next set of keywords relates to the solver step which finds the vibrational states. The results always include VSCF and cc-VSCF (SCF and non-degenerate MP2-like solutions). Use of the restart option makes comparing the solvers very fast, compared to the time to generate the electronic potential energy surface's points. VDPT = option to use 2nd order degenerate perturbation theory, based on the ground and singly excited vibrational levels. Results for virtual CI within the same singly excited space will also be given. Selection of VDPT turns VCI on, as well. (default=.FALSE.) VCI = option to use the virtual CI solver within a space of the ground and both singly and doubly excited vibrational levels. Selection of VCI turns VDPT off. (default=.FALSE.) The solver always finds the ground vibrational state (v=0) by default, and defaults to finding the fundamentals (v=1 in every mode). It can rapidly find excited levels (such as all v=2) if restarted (see READV) from $VIBSCF, using the following to control the excitation levels: IEXC = 1 obtain fundamental frequencies (default) = 2 instead, obtain first overtones = 3 instead, obtain second overtones IEXC2 = 0 skip combination bands (default) = 1 add one additional quanta in other modes = 2 add two other quanta in one mode at a time. IEXC IEXC2 for H2O, which has only three modes: 0 0 only 000 ground state, no transitions 1 0 000, and 100, 010, 001 (fundamentals) 2 0 000, and 200, 020, 002 (1st overtones) 3 0 000, and 300, 030, 003 (2nd overtones) 1 1 000, and 100, 010, 001, 110, 101, 110 (1st overtones and combinations) 1 2 000, and 100, 010, 001, 210, 201, 021 2 1 000, and 200, 020, 002, 120, 102, 012 between them, 1st and 2nd overtones, and all 2-1-0 combinations. ICAS1, ICAS2 = starting and ending vibrations whose quanta are included. The default is all modes, ICAS1=1 and ICAS2=3N-6 (or 3N-5). SFACT = a numerical cutoff for small contributions in the solver. The default is 1d-4: 5d-3 or 1d-3 may affect accuracy of results, 1d-4 is safer, and 1d-5 might not converge. VCFCT = scaling factor for pair-coupling potential. Sometimes when pair-coupling potential values are larger than the corresponding single mode values, they must be scaled down. It is seldom necessary to select a scaling other than unity. (Default=1.0) * * * * * The next two relate to simplified intensity computation. These simplifications are aimed at speeding up MP2 runs, if one does not care so much about intensities, and would like to eliminate the considerable extra time to compute MP2- level dipoles. DMDR must not be used if overtones are being computed. DMDR = if true, indicates that the harmonic dipole derivative tensor $DIPDR will be read and used, rather than computing dipoles. (default=.FALSE.) MPDIP = If .TRUE. the run will compute MP2 level dipoles for the IR intensity evaluation. Entering .FALSE. uses SCF level dipoles instead. Default=.TRUE. for MP2 runs, except when using the RI-MP2 program, which cannot compute MP2 dipoles, and so chooses .FALSE. here. It is more accurate to use the DMDR flag instead instead of turning off MPDIP, if an MP2 level $DIPDR is available from the MP2 hessian run. * * * * These relate to the initial harmonic mode generation. Normally, a $HESS is provided, from which harmonic modes are obtained. It is possible to give the harmonic data explicitly with the first two: RDFRQ = array of harmonic frequencies, starting from the smallest. CMODE = array of normal mode displacements given in the same order as the frequencies read in RDFRQ. The data should be the x,y,z displacement of the first atom of the first mode, then x,y,z for the second atom, then going on to give each additional mode. PROJCT = controls the projection of the hessian matrix (same meaning as in $FORCE). Default is .TRUE. which removes small mixings between rotations or translations and the harmonic modes. * * * * READV = flag to indicate restart data $VIBSCF should be read in to resume an interrupted calculation, or to obtain overtones in follow-on runs. (default is .FALSE.) GEONLY = option to generate all points on the potential energy surface needed by the VSCF routine, without energy evaluations. The purpose of this is to prepare a set of geometries at which the energy is needed. A possible use for this is to obtain energies from a different program package, which might have an energy unavailable in GAMESS, but which lacks its own VSCF program. (default=.false.) ========================================================== $VIBSCF group (optional, relevant to RUNTYP=VSCF) This is restart data, as written to the disk file RESTART in a complete or partially completed previous run. Append a " $END", and also select READV=.TRUE. to read the data. $VIBSCF's contents are different for PETYP=DIRECT or QFF. The format of this group changed in December 2006, so that old groups can no longer be used. ==========================================================

generated on 7/7/2017