The Kawamura potential is commonly used for UO$_2$, see this article.

The notation for this potential is rather unusual. There is only one function for a binary system, where one usually has three potentials. potfit essentially needs three functions for “ntypes = 2”. However, there is an elegant solution to this problem, using global parameters.

To define a proper “kawamura” potential for UO$_2$ one can use the following potential file:

kawamura.pot

#F 0 3
#T PAIR
#C U O
#I 0 0 0
#E
 
global 12
z_U    1.86    1       3
z_O    -1.2    -3      -1
f0     4.186   1.0     10.0
a_U    0.54    0.1     2.5
a_O    1.26    0.1     2.5
b_U    0.16    0.1     0.4
b_O    0.16    0.1     0.4
c_U    0       0       50
c_O    20      0       50
D      18      0       50
Beta   1.25    0       5
r0     1.61    1.4     1.8
 
type kawamura
cutoff 7.0
z_U!
z_U!
f0!
a_U!
a_U!
b_U!
b_U!
c_U!
c_U!
 
type kawamura_mix
cutoff 7.0
z_U!
z_O!
f0!
a_U!
a_O!
b_U!
b_O!
c_U!
c_O!
D!
Beta!
r0!
 
type kawamura
cutoff 7.0
z_O!
z_O!
f0!
a_O!
a_O!
b_O!
b_O!
c_O!
c_O!

While this looks rather cumbersome, it will optimize only the 12 global parameters, which are shared among the three interactions in the binary system.