Towards a Coulomb gas of instantons in the SO(4)xU(1) Higgs model on R_4
  The $SO(4)\times U(1)$ Higgs model on $\R_4$ is extended by a $F^3$ term so that the action receives a nonvanishing contribution from the interactions of 2-instantons and 3-instantons, and can be expressed as the inverse of the Laplacian on $\R_4$ in terms of the mutual distances of the instantons. The one-instanton solutions of both the basic and the extended models have been studied in detail numerically. 
K. Arthur, G.M. O'Brien and D.H. Tchrakian