The repulsive/dispersive term can be evaluated by direct space summation of Equation ( ) over all atom sites and molecules in the central unit cell, 0, and all periodic images of the central unit cell, l, within a valid summation cutoff. The parameters used for the interaction between site 0Ii and lJj are typically determined from the atom type of each participating site. This ensures that species of the same type are treated identically and that the number of parameters does not become overwhelming. It has also been common in CSP to employ combining rules, whereby parameters describing the interactions between atoms of different types are estimated as a geometric or arithmetic average of their homoatomic atom-atom interactions. A geometric mean is often used for A Ii,Jj and C Ii,Jj , whereas an arithmetic mean is usually chosen for the inverse of B Ii,Jj :