# [MMTK] Question about neutron scattering in nMoldyn

Jose Borreguero borreguero at gmail.com
Fri Oct 17 14:04:40 UTC 2008

Dear Konrad,
I take this thread to ask a question I have on the orientational q-average.
Why does nMoldyn use a numerical q-average instead of the analytical one? I
mean:
\integral_{\Omega_q} d\Omega_q exp^{i\vec[q]\vec[r]} = sin(\norm[q]
\norm[r]) / (\norm[q] \norm[r]),
where d\Omega_q is the element of q-solid-angle.
-Jose

On Thu, Oct 16, 2008 at 2:21 PM, Konrad Hinsen <hinsen at cnrs-orleans.fr>wrote:

> On 16.10.2008, at 18:48, Ramon Crehuet wrote:
>
>  trajectory in the low q-range. Apart from the q values, you have to
>> specify the "Q shell width". In the manual is is said that it fixes the
>> q-resolution, but I do not understand the meaning. Is this parameter
>> related to the energy resolution of the neutron spectrometer? If so,
>> which is this relationship?
>>
>
> Every structure factor value is calculated as an orientational average over
> q-vectors with the same length - or rather almost the same length. In
> practice, the average is calculated over q vectors from a spherical shell
> whose mean radius is given by the q value and whose thickness is given by "Q
> shell width".
>
> The Q width parameter exists mostly for computational reasons: in a
> periodic system, except for very high Q values, it is not feasible to pick Q
> vectors of exactly the same length because there are not enough of them. But
> it can also be identified with the angular (not the energetic) resolution of
> the neutron spectrometer.
>
> Hope this helps,
> --
> ---------------------------------------------------------------------
> Centre de Biophysique Moléculaire, CNRS Orléans
> Synchrotron Soleil - Division Expériences
> Saint Aubin - BP 48
> 91192 Gif sur Yvette Cedex, France
> Tel. +33-1 69 35 97 15
> E-Mail: hinsen at cnrs-orleans.fr
> ---------------------------------------------------------------------
>
>
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