Feature: set relax_nmax=0 to enable dry run (#5595)

* Feature: set relax_nmax=0 to enable dry run

* update docs

Author: YuLiu98

docs/advanced/input_files/input-main.md

@@ -1389,7 +1389,7 @@ These variables are used to control the geometry relaxation.
 ### relax_nmax
 
 - **Type**: Integer
-- **Description**: The maximal number of ionic iteration steps, the minimum value is 1.
+- **Description**: The maximal number of ionic iteration steps. If set to 0, the code performs a quick "dry run", stopping just after initialization. This is useful to check for input correctness and to have the summary printed.
 - **Default**: 1 for SCF, 50 for relax and cell-relax calcualtions
 
 ### relax_cg_thr

source/module_io/read_input_item_relax.cpp

@@ -54,11 +54,14 @@ void ReadInput::item_relax()
                 = {"scf", "nscf", "get_S", "get_pchg", "get_wf", "test_memory", "test_neighbour", "gen_bessel"};
             if (std::find(singlelist.begin(), singlelist.end(), calculation) != singlelist.end())
             {
-                para.input.relax_nmax = 1;
+                if (para.input.relax_nmax != 0)
+                {
+                    para.input.relax_nmax = 1;
+                }
             }
             else if (calculation == "relax" || calculation == "cell-relax")
             {
-                if (para.input.relax_nmax == 0) // default value
+                if (para.input.relax_nmax < 0)
                 {
                     para.input.relax_nmax = 50;
                 }

source/module_io/test_serial/read_input_item_test.cpp

@@ -608,9 +608,17 @@ TEST_F(InputTest, Item_test)
         it->second.reset_value(it->second, param);
         EXPECT_EQ(param.input.relax_nmax, 1);
 
+        param.input.relax_nmax = 0;
+        it->second.reset_value(it->second, param);
+        EXPECT_EQ(param.input.relax_nmax, 0);
+
         param.input.calculation = "relax";
         param.input.relax_nmax = 0;
         it->second.reset_value(it->second, param);
+        EXPECT_EQ(param.input.relax_nmax, 0);
+
+        param.input.relax_nmax = -1;
+        it->second.reset_value(it->second, param);
         EXPECT_EQ(param.input.relax_nmax, 50);
     }
     { // out_stru

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 = 0;        ///< number of max ionic iter
+    int relax_nmax = -1;       ///< 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)