Convergence-control.md

@@ -39,10 +39,30 @@ E_pairext: NEW=               0 OLD=               0 DIFF=               0 CONVS
 ```
 
 # Number of iterations
-TODO
+The algorithm iterates until one of the following criteria is met:
+1. convergence is achieved (see the section above);
+2. number of iteration exceeded maximal allowed number of iteration specified in *input* file
+```bash
+maxiters                200     # maximum number of iterations, default=10000
+```
 
 # Imposing constraints to speed up convergence
-TODO
+Providing additional information about properties of the expected solution (based on physical arguments) may speed-up converging significantly. 
+```bash
+spinsymmetry            1       # set to 1 if you want to impose Na=Nb, default: 0
+nocurrents              1       # set to 1 if you want to impose ja=jb=0 
+                                # (solution has no currents), default: 0
+```
 
 # Mixing types
-TODO
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+The self-consistent algorithm tracks the behavior of solutions as a function if the iteration number and utilizes this information to create a new prediction for the solution. In the samples scheme (called *linear mixing*) this process is realized via the formula:
+```c
+for(ixyz = 0; ixyz < SOLDIM; ixyz++) 
+  h_solution[ixyz] = md.linearmixing * h_solution[ixyz] + (1.0-md.linearmixing) * h_solution_old[ixyz];
+```
+Here there is subtle issue: what should be treated as the solution vector: [densities of potentials](Physical quantities)? In principle, this should not matter, but in practice, it turns out that converges properties depend on this choice. For this reason W-SLDA toolkit provided a tag in the *input* file that can be used to select meaning of the *solution* vector:
+```bash
+mixingtype              p   # 'd' - mix densities, 'p' - mix potentials (default)
+```  
+
+Our tests show that typically the algorithm converges faster when *potentials* are used for the mixing procedure. However, we found (rare) cases where mixing of *densities* provides better efficiency. 
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