It is a great success that the application of Morse/Long-Range (MLR) in van der Waals (vdW) complex. MLR is devloped by R. J. Le Roy et al.. You may want to use it when building you own bi-atomic system model. That's cool and you can always use betaFIT, where you can get the code and manual.

In the past, MLR had difficulty in short range (as its name, it focused on long range). It means that you can use a inverse power model to represent the long range behaviour of a system, like when the two monomers in the complex distants away by 10 Angstrom or more. But in long range, the coefficients of the inverse power terms tend to be 0 for the electrons there merge together.

Luckily, Prof Le Roy came up with the damping function (Yeppy!) which can fit this decrease when the nuclear distance vanishes. If you wanna get the details, please read Mol. Phys., 109, 435 (2011). By the way, this modified damping function has already been included in the newest version of betaFIT.

Recently.

Acctually today, I enhanced the MLR in Multi Dimension system by adding a Modified Tang-Tonnies damping function with the paramenter in it as a function of angles. By this way, I have managed the fitting error one the third of the previous model. I will going to talk about this topic in this years ISMS in UIUC.