Least-Squares-Refinement

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A short introdution from Derek Wann and Stuart Hayes:


Least-squares processes, esd’s and correlation


1. Least-squares processes


Essentially a least-squares process involves minimising the weighted sum of the squares of differences between observed and calculated variables, f(i):


Image002.gif or


Image004.gif, where di are differences.


2. Linear least squares


Consider the fitting of a polynomial to a set of n observations, y. In general the following is approximately true,


Image006.gif


However, the following equation is exactly true, although on its own it cannot be solved:


Image008.gif


In order to solve it we require simultaneous equations, which are formed by multiplying throughout by xi:


Image010.gif


Image012.gif


Image014.gif


In matrix notation this is:


Image016.gif = Image018.gifImage020.gif


and can also be written:


Image018.gif = Image016.gifImage023.gif


This is now solvable by calculating the inverse of the matrix. It gives the values of a, b and c that fit the observations best (i.e. with the least sum of squares of differences).


3. Non-linear least squares


We have just seen how to solve a linear least-squares problem. Hovever, in reality many solutions cannot be obtained in this way, because the equations are non-linear. They must be solved by an iterative process, in which we derive shifts in parameters, which take us closer to the optimum solution. This involves first determining the derivatives of the observations with respect to the parameters, which may be done analytically or numerically, depending on whether the equation can be conveniently differentiated. In other words we gain information about how much one observable changes when another is changed.


The mathematics of the matrix manipulations is not given here – but as before, it involves a matrix inversion. From the results of this procedure we obtain:


· parameter shifts – giving better estimates of their values, ready for another cycle of refinement.


· an error matrix, which gives estimated standard deviations and correlations between refining parameters.


4. ESDs


The estimated standard deviation (esd) for the mean of n observations of some parameter is Image025.gif.


Estimated standard deviations for refining parameters are similarly given by the error matrix.


5. R factor


We now need a way of gauging how well we are doing at minimising the differences between the observed and calculated parameters. However, Image004.gif is different depending on the magnitude of what is being measured and so comparison is not possible. For that reason we use the following, normalising the difference term, and comparing it to the absolute values of the measurements (intensities in electron diffraction):


Image027.gif


6. Data correlation


Scattering intensities lie on smooth curves, and we usually have observations at fixed intervals (of s). If we halved this interval, we would have more data, so our esd’s would go down – and down, and down, and down etc. We do two things about this. First, the intervals are chosen so that the maximum information is extracted from the experimental data. This is a matter of trial and error, or now of experience. Secondly, we determine a correlation parameter for a data set, which is a measure of the extent to which any data point (or more precisely, any difference) can be predicted from its immediate neighbours. For perfect correlation, the value is +0.5000000…, –0.5000 would represent perfect negative correlation, i.e. an oscillation in differences (rather unlikely). Random differences, which is what we should have, give a value of zero. In practice anything to ±0.3 is random. If we never get values larger than this, maybe our interval between data points is too large; if we always get 0.49…, the interval is too small.


Once we have the correlation parameter, we use it so that account is taken of the correlation in the data analysis. We use the weight matrix W:


Image029.gif


where the immediately off-diagonal points, X, are:


Image031.gif


The diagonal terms wii are the weighting function for the data set, rising from 0 to 1 from smin to sw1, then 1 to sw2, and then dropping to 0 again at smax.


What we then minimise in the refinements is not Image033.gif = Image004.gif, but D′WD (which is identical to what we used before, if the off-diagonal terms are zero). The R factor is then Image035.gif, where D is the set (vector) or differences and I the set of intensities.


7. Least-squares correlation


Two parameters may be correlated. For example, consider overlapping peaks in a radial distribution curve from two similar distances. If one distance is too large then the other becomes too small in order to compensate. These two parameters will therefore be negatively correlated – one goes up if the other goes down.


If we also refine the average amplitude and this value is too big (the peak is too wide) then the shorter distance will lengthen and the longer one will shorten, so that the two sub-peaks overlap more, and thus make the overall peak narrower again. So the mean amplitude of vibration would be positively correlated with one distance and negatively correlated with the other one.


We can use values from the correlation matrix to see how other parameters should shift if we change the value of one parameter. For example, if the correlation between par(a) and par(b) is 0.3, and par(a) is changed by δa, then:


Image037.gif


The least-squares correlation matrix comes from the same calculation as the esd’s.