Arctangent Approximation to the Intermolecular Potential

The form V̄AB(R)=-(CABR⁶) [2π^-1invtan(d/R)] is proposed as an approximation to the retarded van der Waals potential VAB(R) acting between atoms or molecules A and B. Here CAB is the London-van der Waals constant and d=23αEAαEBc8CAB, with the αE's denoting static electric polarizabilities. V̄AB is shown to be in excellent agreement with available numerical calculations of VAB(R) based on the Casimir-Polder formula, which requires infinite summation over excited atomic states. A heuristic explanation for the validity of the formula is given.