Okay, so I have an expression for V(R) in atomic units (bohrs, hartrees, etc), where V is an energy. I'm assuming this means that if I plug in a value for R (in bohrs), I get back an energy (in hartrees). But if I take the SECOND derivative of V with respect to R, and I want to convert that to SI units, what sort of conversion factors do I need to bring out, and when? For example, if I just take V''(R), leaving everything in atomic units, and plug in R (in bohrs), I get a number...but what units is it in? How would I convert that number to its SI equivalent, which is J/m^2? Of course, V''(R) = d^2 V / dR^2 is supposed to be a (harmonic) force constant, which in SI would be measured in J/m^2. But I've tried a number of different approaches to doing the right sort of conversions, but each one gives me a (wildly) different answer.