XCrySDen can visualize vector-field by arrows. Technically this is noting else than the extension of the display of forces acting on atoms. For this reason the vector field is specified very similar as atomic forces inside the XSF file. All that is needed to specify the forces acting on atoms in the XSF file is to supplement the appropriate Cartesian coordinates (ATOMS or PRIMCOORD section) with the three Cartesian force components:

 AtomicSymbol   X Y Z   Fx Fy Fz

But for the vector fields, the dummy atoms (labeled as "X") are used. Namely, the dummy atoms does not have chemical bonds, hence a large number of dummy atoms can be locates in the close region.

The display parameters of the vector's field arrows can be tuned via the Modify->Force Settings menu (read more), while the display of the dummy "atoms" can be modified via the Modify->Atomic Radius (read more), and Modify->Atomic Color (read more) menus for the radius and color, respectively.

Here is a dummy example of the specification of the vector field in the XSF file (first the atomic structure is specified, and then the vector field by the dummy "X" atoms):

 
ATOMS
  C    0.0000000000    0.0000000000    0.0000000000
  O    0.0000000000    0.0000000000    1.2614009982
  N    0.0000000000    1.1482466623   -0.6997899990
  N    0.0000000000   -1.1482466623   -0.6997899990
  H    0.0000000000    2.0265496485   -0.2028169999
  H    0.0000000000   -2.0265496485   -0.2028169999
  H    0.0000000000    1.1340804828   -1.7049749982
  H    0.0000000000   -1.1340804828   -1.7049749982
  X   2.000  0.000  2.000     0.010  0.000  0.000
  X   1.902  0.618  2.000     0.010  0.003  0.000
  X   1.618  1.176  2.000     0.008  0.006  0.000
  X   1.176  1.618  2.000     0.006  0.008  0.000
  X   0.618  1.902  2.000     0.003  0.010  0.000
  X  -0.000  2.000  2.000    -0.000  0.010  0.000
  X  -0.618  1.902  2.000    -0.003  0.010  0.000
  X  -1.176  1.618  2.000    -0.006  0.008  0.000
  X  -1.618  1.176  2.000    -0.008  0.006  0.000
  X  -1.902  0.618  2.000    -0.010  0.003  0.000
  X  -2.000 -0.000  2.000    -0.010 -0.000  0.000
  X  -1.902 -0.618  2.000    -0.010 -0.003  0.000
  X  -1.618 -1.176  2.000    -0.008 -0.006  0.000
  X  -1.176 -1.618  2.000    -0.006 -0.008  0.000
  . . . 

And here is how XCrySDen displays the vector field (this is merely an "artistic" example):