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$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.
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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.
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generated on 7/7/2017