$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                             
    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