Hi,
In my opinion this distinction is hard to make (strictly, by "ab initio" I
understand the determination of structure of 'very small' molecules with quantum
mechanics methods, SCF/MO.., calculations).
It is normal to make assumptions based on chemical info/common sense even in
what is usually called "ab initio" structure determination. When you determine a
structure, often you make assumptions (for instance, I quote from your -Le Bail-
SDPDD text for sodium oxalate):
>>>
Atoms 3 and 2 (with peak height 184 and 251) would be respectively a carbon and
an oxygen atom or vice versa. The latter option is more convincing because we
expect the carbon atom to be at larger distance from the sodium atom than the
oxygen should be...
<<<
This observation for 'expected' interatomic distances helped you to make a
decision.
So, in my opinion by "ab initio" structure determination we can include all
methods with known fragments, Monte Carlo, etc. since almost always we use more
information than can be extracted from powder X-ray data (it sounds strange
because there is no "ab initio" anymore...).
Comments welcome,
helping
>Armel
>Trying to keep the fire alive.
N. Dragoe