$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,                                                         
         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.                               
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.                                       
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.                        
                       * * * * *                                                
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                       
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.                                      
$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