Water trimer

This picture you saw on the frontpage shows two-body interactions in water trimer. It is obtained with the FMO method for the quantum chemistry part and a set of the FMOcube utilities (included in the tools set supplied with GAMESS). More precisely, this is the difference of the 2 and 1-body densities from a 2-body FMO-RHF calculation, plotted with the cutoff=0.0005 using Molekel (a free graphics software). Negative areas are blue (that is where the electron density decreases) and positive are grey (where the density increases).

Let us proceed in steps in understanding what mysteries water encapsulates.

The picture above is the density difference of FMO1 (1-body FMO) and free water molecules (i.e., each molecule separately). You can see polarisation of hydrogens and oxygens.

This is the same picture from the front page (FMO2 and FMO1 density difference). You can see hydrogen bond formation (grey sachets) between H2O...H-OH.
If you have become very keen on water trimers, here is another way of plotting this quantity.

This is the difference between FMO2 and free monomers. It shows the sum of the previous two interactions, polarisation and two-body effects. Note FMO2 is known to closely reproduce the full RHF density.

Hopefully this small graphical FMO tutorial made you interested in using our method. You can plot the density for your own very molecule using the tools we provide (for free, naturally, for love of the game, so to speak).

The FMOcube utilities and the function to produce cube files are released in FMO 2.0 and up. The option to produce free monomer properties is included in FMO 3.0 and up.