$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. ==========================================================

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