Reference-scales.md

+[[_TOC_]]
 # General info
 During the computation process W-SLDA codes exploit information about typical scales present in the problem. Precisely, *reference scales* define typical orders of magnitude for computed quantities.  The most important reference scale is:  
 * $`k_F=(3\pi^2 n)^{1/3}`$ - Fermi momentum. 
@@ -11,8 +12,8 @@ Finally, chemical potentials also serve as reference scales for static problems:
 * $`\mu_{\downarrow}`$ - chemical potential is spin-down particles (particles of type `b`).
 
 # Defining reference scales for static calculation 
-### Fermi momentum
-There are following methods of  defining the $`k_F`$ reference scale: 
+## Fermi momentum
+There are the following methods of  defining the $`k_F`$ reference scale: 
 * *automatic*: in each iteration code checks what is maximal density $`n=\max[n_{\uparrow}(\bm{r})+n_{\downarrow}(\bm{r})]`$ and next Fermi momentum is computed as $`k_F=(3\pi^2 n)^{1/3}`$,
 * *via input file*: $`k_F`$ is provided by user in input file. To activate this mode you need to **uncomment** tag `referencekF`:     
 ```bash
@@ -27,8 +28,52 @@ void process_params(double *params, double *kF, double *mu, size_t extra_data_si
     // ... 
 }
 ```
-### Chemical potentials
+## Chemical potentials
 Chemical potentials are adjusted automatically when mode with fixed particle number is executed. For mode with fixed chemical potential see [here](Chemical potentials control). 
 
+## Examples
+### Fermi momentum is fixed by density in the box center
+```c
+// Set kF via process_params function
+void process_params(double *params, double *kF, double *mu, size_t extra_data_size, void *extra_data)
+{
+    // set kF
+    double *dens=(double *)extra_data;
+    if(dens[0]>0.0) // do it only if central density has been computed
+    {
+        (*kF) = pow(3.0*M_PI*M_PI*dens[0],1./3.);
+        if(wsldapid==0) wprintf("# UPDATE OF kF=%f\n", (*kF)); // print to stdout
+    }
+
+}
+
+// here you can extract needed data, like central density
+void modify_potentials(int it, wslda_density h_densities, wslda_potential h_potentials, double *params, size_t extra_data_size, void *extra_data)
+{    
+    // DETERMINE LOCAL SIZES OF ARRAYS (CODE DIMENSIONALITY DEPENDENT)
+    int lNX=h_densities.nx, lNY=h_densities.ny, lNZ=h_densities.nz; // local sizes
+    int ixyz;
+    
+    // take value of density in box center and save it to extra_data
+    ixyz = lNZ/2 + lNZ*lNY/2 + lNZ*lNY*lNX/2;
+    double *dens=(double *)extra_data;
+    dens[0] = h_densities.rho_a[ixyz]+h_densities.rho_b[ixyz];
+}
+
+// since you want to pass data between functions, use extra_data buffer
+size_t get_extra_data_size(double *params)
+{
+    return sizeof(double); // I need buffer for density
+}
+
+// here you initialize the buffer
+int load_extra_data(size_t size, void *extra_data, double *params)
+{
+    double *dens=(double *)extra_data;
+    dens[0]=0.0; // set initial value to zero
+    return 0;
+}
+```
+
 # Defining reference scales for time dependent calculations
 All reference scales are provided together with an initial state, i.e. binary files produced by static codes contain this information. Presently there is no option of changing values for reference scales. 
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