Friedel pair is strictly F(hkl) and F(-h,-k,-l). Bijvoet pair is F(h) and any mate that is symmetry-related to F(-h), e.g., F(hkl) and F(-h,k,-l) in monoclinic.
There are always anomalous differences, though they can be unmeasurably small. Bernie Santarsiero On Thu, June 26, 2008 10:55 am, Bernhard Rupp wrote: > Dear All, > > I wonder about the conventions using Friedel vs Bijvoet pair. > > a) there are no differences. As long as h = -h, it's a Friedel > or a Bijvoet pair. They are the same. > > b) A Friedel pair is any reflection h = -h including hR = -h, i.e. > including centric reflections. > A Bijvoet pair is an acentric Friedel pair, it can carry > anomalous amplitude differences, whereas centric Friedel > pairs invariably cannot. Actually, Bijvoet pairs (acentric > Friedel pairs) invariably do carry anomalous amplitude differences. > There is no such thing as no anomalous scattering. > We may elect to ignore it, only. > > c) of course, this all assumes absence of anisotropic AS. > > def b) seems to be helpful in discussions and make sense given that > absolute > > configuration that needs AS signal is somehow associated with > Bijvoet's > work. > > Are any authoritative answers/conventions/opinions available on that ? > > Thx, BR > > ----------------------------------------------------------------- > Bernhard Rupp > 001 (925) 209-7429 > +43 (676) 571-0536 > [EMAIL PROTECTED] > [EMAIL PROTECTED] > http://www.ruppweb.org/ > ----------------------------------------------------------------- > The hard part about playing chicken > is to know when to flinch > ----------------------------------------------------------------- >
