(spinw-addg)=

# spinw.addg method

## Syntax
  
`addg(obj, matrixIdx, {atomTypeIdx}, {atomIdx})`
  
## Description
  
`addg(obj, matrixIdx, {atomTypeIdx}, {atomIdx})` assigns the
$[3\times 3]$ matrix selected by `matrixIdx` (using either the matrix
label or matrix index) to the magnetic sites selected by `atomTypeIdx`
that can contain a name of an atom or its atom index (see [spinw.atom](#spinw-atom)).
If `atomTypeIdx` is not defined, g-tensor will be assigned to all
magnetic atoms.
  
## Examples
  
The following example will add the $g_1$ diagonal matrix to all magnetic
atoms as anisotropic g-tensor and show the effect of a fourfold axis:
  
```matlab
cryst = spinw
cryst.genlattice('lat_const',[4 4 3],'spgr','P 4')
cryst.addatom('r',[1/4 1/4 1/2],'S',1)
cryst.addmatrix('label','g_1','value',diag([2 1 1]))
cryst.gencoupling
cryst.addg('g_1')
cryst.plot('ionMode','g')
```
 
```{image} /_static/img/generated/spinw_1.png
:alt: cryst.plot(
```
  
## Input Arguments
  
`matrixIdx`
: Either an integer, that selects the matrix
  `obj.matrix.mat(:,:,matrixIdx)`, or a string identical to one
  of the previously defined matrix labels, stored in
  `obj.matrix.label`. Maximum value is $n_{mat}$.
  
`atomTypeIdx`
: String or cell of strings that select magnetic atoms by
  their label. Also can be a vector that contains integers, the index of
  the magnetic atoms in [spinw.unit_cell](#spinw-unit-cell), this will assign the given
  g-tensor to all symmetry equivalent atoms. Maximum value is $n_{atom}$.
  If `atomTypeIdx` is not defined, the given g-tensor will be assigned to
  all magnetic atoms. Optional.
 
`atomIdx`
: A vector that contains indices selecting some of the
  symmetry equivalent atoms. Maximum value is the number of symmetry
  equivalent atoms corresponding to `atomTypeIdx`. If the crystal
  symmetry is higher than $P0$, `atomIdx` is not allowed, since the
  g-tensor for equivalent atoms will be calculated using the symmetry
  operators of the space group. Optional.
  
## Output Arguments
  
The function adds extra entries to the `obj.single_ion.g` matrix.
  
## See Also
  
[spinw](#spinw) \| [spinw.addcoupling](#spinw-addcoupling) \| [spinw.addaniso](#spinw-addaniso) \| [spinw.addmatrix](#spinw-addmatrix)