2,718
edits
ISa: Difference between revisions
Jump to navigation
Jump to search
→An estimate for the overall quality of an experimental setup
| Line 6: | Line 6: | ||
This will give you the upper limit of I/sigma(I) for any reflection in your dataset - even if your crystal is great, all reflections are bound to be worse than that. | This will give you the upper limit of I/sigma(I) for any reflection in your dataset - even if your crystal is great, all reflections are bound to be worse than that. | ||
Why does the command give you such a useful value? It just finds the line " a b " in [[CORRECT.LP]], grabs the values of "a" and "b" from the next line, and prints out 1/sqrt(a*b). The values a and b appear in the formula v(I)=a*(v0(I)+b*I^2) which is used by CORRECT to adjust the variances of the intensities, to match their experimental spread. For strong and well-measured reflections, the variance is dominated by the systematic error that is introduced by any beam or | Why does the command give you such a useful value? It just finds the line " a b " in [[CORRECT.LP]], grabs the values of "a" and "b" from the next line, and prints out 1/sqrt(a*b). The values a and b appear in the formula v(I)=a*(v0(I)+b*I^2) which is used by CORRECT to adjust the variances of the intensities, to match their experimental spread. For strong and well-measured reflections, the variance is dominated by the systematic error that is introduced by any beam /spindle / detector /cryo or other instability or malfunction. For weak reflections, v0(I), the variance from counting statistics, dominates. The value for v(I) that the formula gives, will be higher than v'(I)=a*b*I^2 by an amount a*v0(I). Therefore, I/sigma(I) = I/sqrt(v(I)) will be lower than I/sqrt(v'(I)) = 1/sqrt(a*b) which is what the Unix command prints out. | ||
What might go wrong with this simple measure? Sometimes, e.g. if too few strong reflections exist in the dataset, b might come out negative. In that case the Unix command prints out "nan" which means "not a number" and indicates that it could not calculate the square root of a negative number. | What might go wrong with this simple measure? Sometimes, e.g. if too few strong reflections exist in the dataset, b might come out negative. In that case the Unix command prints out "nan" which means "not a number" and indicates that it could not calculate the square root of a negative number. | ||