EXTERNAL=name

Normally, all semiempirical parameters are taken from the module files within MOPAC. When the supplied parameters are not suitable, as in an element recently parameterized, and the parameters have not yet installed in the user's copy of MOPAC, then the new parameters can be inserted at run time by use of EXTERNAL=<filename>, where <filename> is the name of the file which contains the new parameters. If the file-name contains spaces, then enclose the filename in quotation marks, e.g. EXTERNAL="E:\Test Data\Parameter Set". 

<filename> contains a series of parameter definitions in the format:

<Parameter> <Element> <Value of parameter>
where the possible parameters are
USS, UPP, UDD, ZS, ZP, ZD, BETAS, BETAP, BETAD, GSS, GSP, GPP, GP2, HSP, F0SD, G2SD, ALP, XFAC_xx, ALPB_xx, FNnm, n=1,2, or 3, and m=1 to 10, and the elements are defined by their chemical symbols, such as Si or SI.  So FN11 is the first "a" parameter of the core-core term, FN12 is the second "a" parameter (if it exists), FN21 is the first "b" parameter of the core-core term, and FN31 is the first "c" parameter of the core-core term.

When new parameters for elements are published, they can be typed in as shown. This file is ended by a blank line, the word END or nothing i.e., no end-of-file delimiter. An example of a parameter data file is given in the Figure:

Figure: Use of EXTERNAL=name

 
    USS      Si    -34.08201495
    UPP      Si    -28.03211675
    UDD      Si     -6.0
    BETAS    Si     -5.01104521
    BETAP    Si     -2.23153969
    Betad    Si     -1.0
    ZS       Si      1.28184511
    ZP       Si      1.84073175
    ALP      Si      2.18688712
    GSS      Si      9.82
    GPP      Si      7.31
    GSP      Si      8.36
    GP2      Si      6.54
    HSP      Si      1.32
    BETAD    TC      -5.72433498
    F0SD     TC      5.43488602
    G2SD     TC      1.10687502
    XFAC_Ag   I      2.0
    ALPB_C   Rb      2.0

Derived parameters do not need to be entered; they will be calculated from the optimized parameters. All "constants" such as the experimental heat of atomization are already inserted for all elements.

If a parameter is not set, e.g. UPP for hydrogen, do not attempt to define the parameter as zero.  If a parameter that has not been defined is given a defined value of zero, then it will become defined, and have the value zero.  From that point on, it would be recognized as a defined parameter, usually with disastrous results.  So do not use lines such as:

 
    UPP       H    0.0
   

Derived parameters do not need to be entered; they will be calculated from the optimized parameters. All "constants" such as the experimental heat of atomization are already inserted for all elements.