Mark,
In the small-molecule crystal structures I work with it's relatively
common to see localized difference electron density along covalent bonds
or in the places you'd expect to see lone pairs during refinement after
you've fit and modeled the atoms reasonably well and the phases are
pretty good. It's usually not as strong as difference density for
hydrogens, before you put them in, but it's often pretty clearly visible
once you have.
(I use SHELXLE as an interface for small molecule refinements because of
a somewhat Coot-like experience in viewing maps).
Phil Jeffrey
Princeton
What you CAN do in fact is appropriately subtract "spherical electron
density" from the experimental density and see what is left (i.e.
directional ED that is 'surplus'). I tried to quickly find a paper on
that, they exist, and they show that experimental density does confirm
what we learn in chemistry class, orbitals are not imaginary.
Mark
