Fact-checking scientific publications

Occasionally, when reading a scientific paper I'll sit down and try to rederive their results. My intention is not to try to expose or embarrass fellow scientists but rather it is a humble exercise to verify that I have understood the basic principles invoked by the authors, and that my memory hasn't eroded too much in the last half-decade or so. 

Most recently, I came across an AFM/STM study of a pair of charged molecules sitting on a surface. The authors used a simple model of point charges and their image charges to estimate the electrostatic energy of the system. Simple enough, it's just 1st year electrostatics, I thought. I placed my two point charges at the distance reported in the paper, gave each a charge of one electron, and added up all of the pairwise interactions. 

Somehow, no matter how I varied the molecule-surface distance, my result was always half an order to a full order of magnitude larger! See for yourself, and let me know if I got something wrong. Below: excerpt from the original article followed by my sketch of the system.