Docs: Create soc.md (#1325)

* Create soc.md

* Update docs/advanced/scf/soc.md

Co-authored-by: Chun Cai <amoycaic@gmail.com>

docs/advanced/scf/soc.md

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+## SOC Effects
+### SOC 
+`lspinorb` is used for control whether or not SOC(spin-orbit coupling) effects should be considered.
+
+Both `basis_type=pw` and `basis_type=lcao` support `scf` and `nscf` calculation with SOC effects.
+
+Atomic forces and cell stresses can not be calculated with SOC effects yet. 
+
+### Pseudopotentials and Numerical Atomic Orbitals
+For Norm-Conserving pseudopotentials, there are differences between SOC version and non-SOC version.
+
+Please check your pseudopotential files before calculating.
+In `PP_HEADER` part, keyword `has_so=1` and `relativistic="full"` refer to SOC effects have been considered, 
+which would lead to different `PP_NONLOCAL` and `PP_PSWFC` parts.
+Please be careful that `relativistic="full"` version can be used for SOC or non-SOC calculation, but `relativistic="scalar"` version only can be used for non-SOC calculation.
+When full-relativistic pseudopotential is used for non-SOC calculation, ABACUS will automatically transform it to scalar-relativistic version.
+
+Numerical atomic orbitals in ABACUS are unrelated with spin, and same orbital file can be used for SOC and non-SOC calculation.
+
+### Partial-relativistic SOC Effect
+Sometimes, for some real materials, both scalar-relativistic and full-relativistic can not describe the exact spin-orbit coupling. 
+Artificial modulation can help for these cases.
+
+`soc_lambda`, which has value range [0.0, 1.0] , is used for modulate SOC effect.
+In particular, `soc_lambda 0.0` refers to scalar-relativistic case and `soc_lambda 1.0` refers to full-relativistic case.