add some docs and comments

Author: WHUweiqingzhou

docs/advanced/input_files/input-main.md

@@ -88,6 +88,8 @@
     - [scf\_thr](#scf_thr)
     - [scf\_ene\_thr](#scf_ene_thr)
     - [scf\_thr\_type](#scf_thr_type)
+    - [scf\_thr\_os](#scf_thr_os)
+    - [scf\_os\_ndim](#scf_os_ndim)
     - [chg\_extrap](#chg_extrap)
     - [lspinorb](#lspinorb)
     - [noncolin](#noncolin)
@@ -1205,6 +1207,21 @@ Note: In new angle mixing, you should set `mixing_beta_mag >> mixing_beta`. The
 
 - **Default**: 1 (plane-wave basis), or 2 (localized atomic orbital basis).
 
+### scf_thr_os
+
+- **Type**: double
+- **Description**: The slope threshold to determine if the SCF is stuck in a charge density oscillation. To this end, Least Squares Method is used to calculate the slope of the logarithmically taken drho for the previous `scf_os_ndim` iterations. If the calculated slope is larger than `scf_thr_os`, stop the SCF.
+  - **>=0**: The SCF will continue to run regardless of whether there is oscillation or not. 
+  - **<0**: If the calculated slope is larger than `scf_thr_os`, stop the SCF.
+
+- **Default**: 0
+
+### scf_os_ndim
+
+- **Type**: int
+- **Description**: To determine the number of old iterations' `drho` used in slope calculations.
+- **Default**: `mixing_ndim`
+
 ### chg_extrap
 
 - **Type**: String

source/module_elecstate/module_charge/charge_mixing.cpp

@@ -1731,6 +1731,7 @@ bool Charge_Mixing::if_scf_oscillate(const int iteration, const double drho, con
     double slope = 0.0;
 
     // Least Squares Method
+    // this part is too short, so I do not design it as a free function in principle
     double sumX = 0, sumY = 0, sumXY = 0, sumXX = 0;
     for (int i = iteration - iternum_used; i < iteration; i++)
     {