$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
quadrupole.
 
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
         computed.
 
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.