Fix: fix a bug of relax_nmax default (#5506)

* fix a bug of relax_nmax default

* forbid relax_nmax=1 if relax

* Revert "forbid relax_nmax=0 if relax"

This reverts commit 00409db.

* Revert "fix a bug of relax_nmax default"

This reverts commit 86af7fc.

* the default value of relax_nmax as 0

* add some comment

* docs

Author: WHUweiqingzhou

docs/advanced/input_files/input-main.md

@@ -1389,7 +1389,7 @@ These variables are used to control the geometry relaxation.
 
 - **Type**: Integer
 - **Description**: The maximal number of ionic iteration steps, the minimum value is 1.
-- **Default**: 1
+- **Default**: 1 for SCF, 50 for relax and cell-relax calcualtions
 
 ### relax_cg_thr
 

source/module_io/read_input_item_relax.cpp

@@ -58,7 +58,7 @@ void ReadInput::item_relax()
             }
             else if (calculation == "relax" || calculation == "cell-relax")
             {
-                if (!para.input.relax_nmax)
+                if (para.input.relax_nmax == 0) // default value
                 {
                     para.input.relax_nmax = 50;
                 }

source/module_parameter/input_parameter.h

@@ -152,7 +152,7 @@ struct Input_para
     bool relax_new = true;
     bool relax = false; ///< allow relaxation along the specific direction
     double relax_scale_force = 0.5;
-    int relax_nmax = 1;        ///< number of max ionic iter
+    int relax_nmax = 0;        ///< number of max ionic iter
     double relax_cg_thr = 0.5; ///< threshold when cg to bfgs, pengfei add 2011-08-15
     double force_thr = -1;     ///< threshold of force in unit (Ry/Bohr)
     double force_thr_ev = -1;  ///< threshold of force in unit (eV/Angstrom)