Simulation of Van der Waals forces using Comsol v4.3

I'm trying to simulate the attractive Van der Waals force between two nanometric structures. My initial idea was to integrate the Lennard-Jones potential created by one domain (\integral (rho/M)*(1/|r1-r2|^8-1/|r1-r2|^14)*(r1-r2)*dxdydz, however I'm not able to integrate functions that have more variables (i.e. the result of integrating this over R1 would be a function of R2). What suggestions would you have to approach this problem?

Any help would be appreciated!

Marc