# Punishments for Analytic Potentials

For analytic potentials it may sometimes be necessary to use additional punishments to keep the potential in a predefined parameter space. If the initial contraints, given in the parameter file can not achieve this, then there is a special function which is called before every force calculation. This function can be used to check for additional contraints on analytic potentials.

The function is called `apot_punish`

and can be found in `functions.c`

.

/**************************************************************** * * punish analytic potential for bad habits * ****************************************************************/ real apot_punish(real *params, real *forces)

The parameters are

**real *params**

This is the array with the new parameters, access it with `params[idx[i]]`

, where $i<\text{ndim}$.

**real *forces**

This is the global forces array, store additional punishments to `forces[punish_par_p+ i]`

, where $i<\text{ndim}$

Here is an example, where a function is punished if two parameters are too close together:

/* punish m=n for universal embedding function */ if (strcmp(apot_table.names[i], "universal") == 0) { x = params[j+ 2] - params[j+ 1]; if (fabs(x) < 1e-6) { forces[punish_pot_p+ i] = apot_punish_value / (x * x); tmpsum += apot_punish_value / (x * x); } }

The return value is used to make *potfit* aware of these extra punishments.