Format of the params file for parameterization

The params file contains the parameters you would like to optimize to fit the QM data. An example of this file is ~jkua/reac/sbcl/params

The first number corresponds to the section in the ffield file, the second number to the item in that sub-section, and the third number to the parameter in the sub-section. For example 2 5 6 in the first line corresponds to the atom section (the 2nd section), Sb subsection (Sb is the 5th atom listed), parameter number 6, which is the Sb EEM shielding parameter whose current value is 0.5649. The fourth number is the increment by which the value will be changed, i.e., in this case the optimization will try 0.5649-0.025 and 0.5649+0.025. The fifth and sixth values are the limits of the parameter, i.e., during the optimization this parameter should not increase beyond 1.0 or decrease beyond 0.5.

  2  5  6  0.0250   1.0  0.5    ! Sb EEM shielding
  2  5 14  0.0050   5.0  2.0    ! Sb EEM electronegativity
  2  5 15  0.0050   9.0  2.0    ! Sb EEM hardness
  3 16  1  0.0200  120.0 50.0    ! Sb-Cl sigma-BDE
  3 16  4  0.0500  -0.10 -0.80
  3 16  8  0.1000   0.10  1.00
  3 16  9  0.0200   0.10  15.0
  3 16 13  0.0200   -0.04  -0.10
  3 16 14  0.0200   4.5   8.0

--Relevant sections in ffield--

Sb   2.1746   3.0000 121.7500   2.1646   0.2466   0.5649   0.0535   5.0000
      12.0773  12.9676   4.0000   0.0001   0.0000   3.2800   6.0677   0.0000
       -1.0000   0.0000  63.2000   3.2928   0.5000   0.1542   0.8563   0.0000
       -3.2127   4.0000   1.0338   6.0000   2.5791   0.0000   0.0000   0.0000

5  6  80.0000   0.0000   0.0000   0.2500   0.0000   1.0000   6.0000   0.5000
           0.7500  -0.2500  20.0000   1.0000  -0.1000   7.5000   0.0000   0.0000

--

Let's look at a second example. In the fourth line (3 16 1 etc). 3 is the "Bond" section, 16 is the bond between atoms 5 and 6 which in this case are Sb and Cl, and 1 is the bond dissociation energy parameter currently set at 80.000. The increment is 0.0200 which seems rather small given the limist of 120.0 and 50.0.

The parext keyword in the control file ...

The parsca keyword in the control file allows an overall scaling of the increments (say when finer or coarser changes are required).