First, export cube file of transition density, assume it is named as TD.cub.
Load TD.cub into Multiwfn, then input
13 //Process grid data
11 //Grid data calculation
13 //Get absolute value
17 //Show statistic data of the points in specific spatial and value range
1 //Obtain statistic data for all grid points
Then you will see "X,Y,Z of barycenter (in Bohr)", you may use this point as center to plot the vector.
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