$DAMP group (optional, relevant if RUNTYP=MAKEFP) This group provides control over the screening of the charge term in the distributed multipole expansion used by the EFP model for electrostatic interactions, to account for charge penetration. See M.A.Freitag, M.S.Gordon, J.H.Jensen, W.A.Stevens J.Chem.Phys. 112, 7300-7306(2000) L.V.Slipchenko, M.S.Gordon J.Comput.Chem. 28, 276-291(2007) The screening exponents are optimized by fitting a damped multipolar electrostatic potential to the actual quantum mechanical potential of the wavefunction, computed on concentric layers of united spheres (namely, "GEODESIC" layers for WHERE=PDC in $ELPOT). See $STONE's generation of the unscreened classical multipoles, $PDC's generation of the true quantum potentia, and $DAMPGS. Different multipole damping functions can be generated. The first contains a single exponential form, (1 - beta*exp(-alpha*r)) and the second function is a single Gaussian form, (1 - beta*exp(-alpha*r**2)) The exponent 'alpha' values are optimized (normally with beta=one), with starting values defined in $DAMPGS. The exponential fit is used for fragment-fragment charge penetration screening, while the Gaussian fit is used in ab initio-fragment screening. See equations 28 and 4 in the reference. These two screen only the charge-charge interactions. It is also possible to generate a "higher order exponential" screening term, meaning that in addition to the charge-charge energy, also affects charge-dipole, charge-quadrupole, and dipole-dipole energy terms. Words of advice: 1. Higher order screening is usually similar in accuracy to just charge-charge screening, except in molecules without dipole moment, such as ethylene or benzene. 2. If the bond midpoints have smaller charges, it may be more physically reasonable to screen only the atomic monopoles, see ISCCHG. 3. Use of the numerical Stone distributed multipole analysis may not be fully converged with respect to the level of highest used multipole moment (octapole) and corresponding energy terms (quadrupole-quadrupole), which makes screening much more problematic. 4. Accuracy of screening with the damping function of a single exponential form depends on a region of fitting the quantum mechanical electrostatic potential, i.e., a radius of first sphere with grid points (parameter VDWSCL in $PDC). A general trend is that for molecules with stronger electrostatic interaction, and, consequently, shorter intermolecular separations, e.g., methanol and water, smaller values of VDWSCL are preferable, whereas for weaker interacting molecules, e.g., dichloromethane and acetone, bigger VDWSCL values are more acceptable. Our recommended VDWSCL values are 0.4-0.5 for methanol, 0.5-0.8 for water, and 0.7-0.9 for weaker bonded molecules. Note that VDWSCL values of 1.0 and higher often result in not converged or badly converged damping parameters, and are not recommended. The default VDWSCL value is 0.7. 5. If the non-linear parameters alpha increase to 10, that term is effectively removed from the screening. This happens sometimes with buried atoms, and fairly often with bond mid-points. 6. Double check the numerical results carefully. ISCCHG = 0 use both atoms and bond midpoints as screening centers (the default) 1 use only atoms as screening centers IFTTYP = selects the type of multipole screening fit: 0 means generate a Gaussian fit, for use as SCREEN input in $FRAGNAME. 2 means generate an exponential charge-charge fit, for use as SCREEN2 input in $FRAGNAME. 3 means generate an exponential higher order fit, for use as SCREEN3 input in $FRAGNAME. If you wish to use Gaussian screening for EFP-EFP, simply copy the SCREEN output into a SCREEN1 section. IFTFIX = 0 means the coefficients in the fit (beta) are free parameters 1 means the coefficients are held to unity. In case the linear coefficients become large, and particularly if they are negative, a fit with unit coefficients is more reasonable. The default is to do both fits in one run, IFTTYP(1)=2,0, using unit coefficients, IFTFIX(1)=1,1. The remaining parameters are seldom given: NMAIN = the number of centers to receive a smaller alpha initial value, 2.0, which defaults to the number of atoms. The remaining centers, usually the bond midpoints, receive a larger starting value, 4.0. $DAMPGS gives more control of the values. MAXIT = maximum iterations in the fit, default=30. THRSH = printing threshold for large deviations. The default is 100.0 kcal/mol. ========================================================== ==========================================================

generated on 7/7/2017