Chemistry Reference
In-Depth Information
atom would mean that this atom has the same density shifts as the reference atom,
but it still may have a different monopole value. The parameter
R
-value is defined in
the following equation, where “dip” abbreviates dipole, “quad” quadrupole, “octu”
octupole, and “hexa” hexadecapole.
R
para
¼
R
dip
þ
R
quad
þ
R
octu
þ
R
hexa
;
þl
;
(27)
R
lm
¼
P
lm
;
reference
P
lm
;
model
l
¼
1
;
2
;
3
;
4
:
m¼l
When a multipole refinement is performed, one usually tries to avoid anharmo-
nic nuclear motion refinement, as this might introduce severe parameter correla-
tions. Figure
14
shows the deepest hole and highest peak in the residual density
together with the resulting VSCCs close to the P atom from such a refinement
excluding anharmonic motion.
The residual density peak and hole are closer to the P atom than the VSCCs and
they are lying on a line through the nucleus of the P atom. Due to the close vicinity of
the residual density features and the VSCCs naturally the question arises whether
these are influenced by the peak and hole. We will see that this is indeed the case: two
of the VSCCs are artificial and occur only due to the neglect of present anharmonic
nuclear motion of the P atom. When anharmonic motion is taken into account and the
Gram-Charlier parameters are refined, these artificial VSCCs disappear and the
absolute values of residual density peak and hole become smaller.
Figure
15
compares the final model that takes anharmonic motion into account
and the model that neglects anharmonic motion.
Fig. 14 Largest peak (0.23 e
˚
3
) and deepest hole (
0.21 e
˚
3
), in the residual density (
small
red spheres
), and VSCCs (
white spheres
) in the vicinity of the P atom (
green sphere
) after a
multipole refinement excluding anharmonic motion
