$PDC group             (relevant if WHERE=PDC in $ELPOT)                        
     This group determines the points at which to compute                       
the electrostatic potential, for the purpose of fitting                         
atomic charges to this potential.  Constraints on the fit                       
which determines these "potential determined charges" can                       
include the conservation of charge, the dipole, and the                         
PTSEL  =        determines the points to be used, choose                        
       GEODESIC to use a set of points on several fused                         
                sphere van der Waals surfaces, with points                      
                selected using an algorithm due to Mark                         
                Spackman.  The results are similar to those                     
                from the Kollman/Singh method, but are                          
                less rotation dependent. (default)                              
         CONNOLLY to use a set of points on several fused                       
                sphere van der Waals surfaces, with points                      
                selected using an algorithm due to Michael                      
                Connolly.  This is identical to the method                      
                used by Kollman & Singh (see below)                             
         CHELPG to use a modified version of the CHELPG                         
                algorithm, which produces a symmetric                           
                grid of points for a symmetric molecule.                        
CONSTR = NONE   - no fit is performed.  The potential at                        
                  the points is instead output according                        
                  to OUTPUT in $ELPOT.                                          
         CHARGE - the sum of fitted atomic charges is                           
                  constrained to reproduce the total                            
                  molecular charge. (default)                                   
         DIPOLE - fitted charges are constrained to                             
                  exactly reproduce the total charge                            
                  and dipole.                                                   
         QUPOLE - fitted charges are constrained to                             
                  exactly reproduce the charge, dipole,                         
                  and quadrupole.                                               
    Note: the number of constraints cannot exceed                               
    the number of parameters, which is the number                               
    of nuclei.  Planar molecules afford fewer                                   
    constraint equations, namedly two dipole                                    
    constraints and three quadrupole constraints,                               
    instead of three and five, respectively.                                    
* * the next 5 pertain to PTSEL=GEODESIC or CONNOLLY * *                        
VDWSCL = scale factor for the first shell of VDW spheres.                       
         The default of 1.4 seems to be an empirical best                       
         value. Values for VDW radii for most elements up                       
         to Z=36 are internally stored.                                         
VDWINC = increment for successive shells (default = 0.2).                       
         The defaults for VDWSCL and VDWINC will result                         
         in points chosen on layers at 1.4, 1.6, 1.8 etc                        
         times the VDW radii of the atoms.                                      
LAYER  = number of layers of points chosen on successive                        
         fused sphere VDW surfaces (default = 4)                                
Note: RUNTYP=MAKEFP's screening calculation changes the                         
defaults to VDWSCL=0.5 or 0.8 depending on the type of                          
Stone analysis, VDWINC=0.1, LAYER=25, and MAXPDC=100,000.                       
NFREQ  = flag for particular geodesic tesselation of                            
         points.  Only relevant if PTSEL=GEODESIC.                              
         Options are:                                                           
          (10*h + k)  for   {3,5+}h,k tesselations                              
         -(10*h + k)  for   {5+,3}h,k tesselations                              
         Of course both nh and nk must be less than 10,                         
         so NFREQ must lie within the range -99 to 99.                          
         The default value is NFREQ=30 (=03)                                    
PTDENS = density of points on the surface of each scaled                        
         VDW sphere (in points per square au).  Relevant                        
         if PTSEL=CONNOLLY.  Default=0.28 per au squared,                       
         which corresponds to 1.0 per square Angstrom, the                      
         default recommended by Kollman & Singh.                                
   * * * the next two pertain to PTSEL=CHELPG * * *                             
RMAX   = maximum distance from any point to the closest                         
         atom.  (default=3.0 Angstroms)                                         
DELR   = distance between points on the grid.                                   
         (default=0.8 Angstroms)                                                
MAXPDC = an estimate of the total number of points whose                        
         electrostatic potential will be included in the                        
         fit. (default=10000)                                                   
CENTER = an array of coordinates at which the moments were                      
DPOLE  = the molecular dipole.                                                  
QPOLE  = the molecular quadrupole.                                              
PDUNIT = units for the above values.  ANGS (default) will                       
         mean that the coordinates are in Angstroms, the                        
         dipole in Debye, and quadrupole in Buckinghams.                        
         BOHR implies atomic units for all 3.                                   
  Note: it is easier to compute the moments in the                              
  current run, by setting IEMOM to at least 2 in                                
  $ELMOM.  However, you could fit experimental data,                            
  for example, by reading it in here.                                           
     There is no unique way to define fitted atomic                             
charges.  Smaller numbers of points at which the electro-                       
static potential is fit, changes in VDW radii, asymmetric                       
point location, etc. all affect the results.  A useful                          
bibliography is                                                                 
U.C.Singh, P.A.Kollman, J.Comput.Chem. 5, 129-145(1984)                         
L.E.Chirlain, M.M.Francl, J.Comput.Chem. 8, 894-905(1987)                       
R.J.Woods, M.Khalil, W.Pell, S.H.Moffatt, V.H.Smith,                            
   J.Comput.Chem. 11, 297-310(1990)                                             
C.M.Breneman, K.B.Wiberg, J.Comput.Chem. 11, 361-373(1990)                      
K.M.Merz, J.Comput.Chem. 13, 749(1992)                                          
M.A.Spackman, J.Comput.Chem. 17, 1-18(1996)                                     
Start your reading with the last paper shown.                                   

generated on 7/7/2017