[Xplor-nih] A basic query regarding RDC calculation.

[Charles at Schwieters.org]

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Hello Arun--

> 
> A.x=b ---- eq (1)
> 
> where
> 
> A is a matrix composed of the direction cosines of the internuclear vectors
> ( for the fragment under concern)
> 
> x is the alignment tensor matrix and
> 
> b is the Experimental RDC for the fragment
> 
> First we calculate x by doing an inverse of A and multiplying it with b as
> 
> x = inverse(A) * b

note that A is not a square matrix in general (the rank of the tensor is
usually different than the number of observed RDCs). A more generalized
inverse is possible- this is the idea of the SVD approach:
Losonczi J.A., Andrec M., Fischer M.W.F. and Prestegard J.H. (1999). 
J. Magn. Reson. 138:334\u2013342

> Or is there any loss of information when we do the back calculation for
> expected RDC that brings about the difference between the calculated and
> experimental RDC.

Usually, there are more observed RDCs than tensor degrees of freedom,
such that observed and calculated RDC values can not agree.

hope this helps--
Charles
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