deepak1032 said:
Further, as you correctly analyzed, I'm about to work with London/Vander Waal's forces and thereby hunting in dark to find a way out. The only way I found is to just position (say H2 molecule to be adsorbed) near the other molecule (or substrate) and find out the changes.
Right, well that's pretty much the way you'd do it. Create your model structure, position the adsorbed molecule in some decent guess of its position, do a geometry optimization to find the lowest energy conformation. You'd also want to try out a number of different guesses corresponding to the different binding modes; (e.g. side-on, end-on, etc) even if they're higher in energy you should often be able to find the local energetic minima for the various binding modes.
And so you have your difference in energy between the bound and unbound structure, \Delta E. But note that what you're calculating is the difference in electronic energy. To get a value comparable to experiment, you need to take into account the difference in zero-point vibrational energy, which involves doing a frequency calculation (= second-order derivatives).
So the practice of doing the actual calculation isn't too difficult. But since you've got vdW forces involved, finding a good DFT method isn't that easy; developing functionals that better reproduce dispersion forces is a pretty active research field. At the moment http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JCPSA6000116000022009620000001&idtype=cvips&gifs=yes" which is quite good for reaction energies, is rather infamous for not reproducing vdW interactions
at all. (the molecules just don't bind)
The http://prola.aps.org/abstract/PRB/v45/i23/p13244_1" for adsorption studies. I don't really know how well they stack up against each other though, there's probably a benchmark study or two out there.
Those are functionals you'll probably be able to find in major existing packages (certainly Gaussian03, Jaguar 7). Since it's an active area of research there's a lot of http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000096000014146107000001&idtype=cvips&gifs=yes" out there, but often implemented in less mature programs. (Taking a second here to acknowledge the work of Bengt Lundqvist who's spent as much time as almost anyone developing functionals that get vdW surface adsorption right)
So my best suggestion is to look around using your favorite tool (ISI Knowledge, Google Scholar, etc) and see what benchmark studies you can find on situations/compounds that most resemble what you want to study, and pick your method.
An important point as far as scientific rigor goes, BTW, is that once you do pick a method - stick with it. Don't fall for the temptation to pick whatever method happens to give the best results for in each particular case, or your results will be useless.
