add some docs (#4039)

docs/advanced/scf/converge.md

@@ -10,6 +10,8 @@ For each of the mixing types, we also provide variables for controlling relevant
 
 `mixing_ndim` is the mixing dimensions in DIIS (broyden or pulay) mixing. Gerenally, a larger `mixing_ndim` leads to a better convergence. the default choice `mixing_ndim=8` should work fine in most cases. For `mixing_type`, the default choice is `broyden`, which is slightly better than `Pulay` typically. Besides that, a large `mixing_beta` means a larger change in electron density for each SCF step. For well-behaved systems, a larger `mixing_beta` leads to faster convergence. However, for some difficult cases, a smaller `mixing_beta` is preferred to avoid numerical instabilities.
 
+For most isolated systems, Kerker preconditioning is unnecessary. You can turn off it by setting `mixing_gg0  0.0` to get a faster convergence.
+
 For non-spin-polarized calculations, the default choices usually achieve convergence. If convergence issue arises in metallic systems, you can try different value of Kerker preconditioning [mixing_gg0](../input_files/input-main.md#mixing_gg0) and [mixing_gg0_min](../input_files/input-main.md#mixing_gg0_min), and try to reduce `mixing_beta`, which is 0.8 defaultly for `nspin=1`.
 
 For magnetic calculations, `mixing_beta_mag` and `mixing_gg0_mag` are activated. Considering collinear calculations, you can rely on the default value for most cases. If convergence issue arises, you can try to reduce `mixing_beta` and `mixing_beta_mag` together. For non-collinear calculations, tradtional broyden usually works, especially for a given magnetic configuration. If one is not interested in the energies of a given magnetic configuration but wants to determine the ground state by relaxing the magnetic moments’ directions, the standard Broyden mixing algorithm sometimes fails to find the correct magnetic configuration. If so, we can set [mixing_angle=1.0](../input_files/input-main.md#mixing_angle), which is a promising mixing method proposed by J. Phys. Soc. Jpn. 82 (2013) 114706.