Docs: update initialization.md (#1330)

* Docs: update initialization.md

* Apply suggestions from code review

Co-authored-by: Wenfei Li <38569667+wenfei-li@users.noreply.github.com>

docs/advanced/scf/initialization.md

@@ -1 +1,21 @@
-# Initializing SCF
+# Initializing SCF
+Good initializing would abate the number of iteration steps in SCF.
+Charge density should be initialed for constructing the initial hamiltonian operator. 
+
+In PW basis, wavefunction should be initialized for iterate diagonalization method.
+In LCAO basis, wavefunction can be read to calculate initial charge density. The wavefunction itself does not have to be initialized.
+
+## Charge Density
+`init_chg` is used for choosing the method of charge density initialization.
+ - `atomic` : initial charge density by atomic charge density from pseudopotential file under keyword `PP_RHOATOM` 
+ - `file` : initial charge density from files produced by previous calculations with [`out_chg 1`](../elec_properties/charge.md).
+
+## Wave function
+`init_wfc` is used for choosing the method of wavefunction coefficient initialization.
+
+When `basis_type=pw`, setting of `random` and `atomic` are supported.
+Atomic wave function is read from pseudopotential file under keyword `PP_PSWFC`, if setting is `atomic` and number of band of atomic wavefunction less than `nbands` in INPUT file, the extra bands will be initialed by random.
+
+When `basis_type=lcao`, we further support reading of initial wavefunction by setting `init_wfc` to `file`.
+In LCAO code, wave function is used to initialize density matrix and real-space charge density.
+For such purpose, a file containing wavefunction must be prepared. Such files can be generated from previous calculations with [`out_wfc_lcao 1`](../elec_properties/wfc.md).