Detailed Marking Scheme

Detailed Marking Scheme
2002/2003

School of Crystallography, Birkbeck College, University of London

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Aims & Instructions

Outline Marking Scheme

Category Marks General Comments on Mark Allocation

(a) General Presentation 5 Marks are given for overall appearance, e.g. clean layout with clear headings, tables and pictures in appropriate places, and general attractiveness of the report.

HTML & Navigation 5 Marks are given for ease of navigation (which is easier for single page documents), correct HTML links, and HTML that displays correctly. Marks are deducted if the course organiser has to alter the report so that it displays correctly on the screen. Marks will be deducted for non-generic HTML.

Introduction 5 Report should be identified by author's name. The introduction should set the scene correctly. The student should demonstrate that he or she understands the aims and objectives of the exercise correctly, especially the concept of using subgroups to try lower symmetries.

(b) Data Set A: Strategy 5 Marks are given for a description of the various steps taken. There is no need for a blow-by-blow account, which may be penalised. Marks will be deducted for incorrect strategies including refinement of anisotropic displacement factors, individual isotropic B values, or too many parameters in general.

Tables 10 Full marks will be given for supplying appropriate tables and then marks will be deducted for:

Missing captions
Missing units
Missing esd (su) values
Missing R factors
Missing sections of tables
Wrong parameter or esd values
Excessive parameter or esd values (i.e. too many digits)
Excessive number of tables (start and end tables are sufficient)

Figures 5 A figure should be given showing observed, calculated, and difference profiles together with calculated reflection positions. Marks will be lost for missing figure captions, excessive number (or inapprpriate use) of figures. Completely missing figures will result in zero marks. Figures may be included to show graphical background used. Best fit should show acceptable difference plot.

(c-i) Data Set B: Strategy
(for correct structure) 5 Marks are given for refining the low temperature structure with the correct space group P2(1)/n as demonstrated by achieving a correct value for beta (90.25° or 89.75°). Marks will be deducted for use of anisotropic displacement factors. Individual isotropic B values should be used.

Tables 10 Full marks will be given for supplying appropriate tables and then marks will be deducted for:

Missing captions
Missing units
Missing esd (su) values
Missing R factors
Missing sections of tables
Wrong parameter or esd values
Excessive parameter or esd values (i.e. too many digits)
Excessive number of tables (start and end tables are sufficient)
Wrong site occupancies
Badly split atom in going from O(3) to new O(4)

Figures 5 A figure should be given showing observed, calculated, and difference profiles together with calculated reflection positions. Marks will be lost for missing figure captions, excessive number (or inapprpriate use) of figures. Completely missing figures will result in zero marks. Figures may be included to show graphical background used. Best fit should show acceptable difference plot.

(c-ii) Data Set B: Strategy
(for wrong structure) 5 A very clear explanation of the strategy is required here given that students may either refine in P2(1)/b11 with x unique or P12(1)/a1 with y unique, but with the x and y coordinates swopped and a and b values swopped. No further marks to be deducted for similar errors made in previous refinements.

Table & Figure 3 + 2 Marks are given for supplying appropriate table and figure of best fit using wrong space group. No further marks to be deducted for similar errors made in previous tables.

(d) General Conclusions 6 Marks are to be given for identifying (as opposed to obtaining) the correct solution. The poorer fit relative to that at 300K could be commented on.

300K versus 5K 6 Students should mention the following to achieve full marks: changes in lattice parameters, thermal parameters, and the monoclinic distortion that results.

Data Set A versus B 6 The key point here is the difference between low- and high-resolution (in terms of d-spacing) data sets and the consequences of this, in particular the restricted use of parameters (i.e. a limited number) when the data set is limited. In addition, the fact that low resolution structures cannot distinguish between orthorhombic symmetry and a monoclinic distortion. The figure showing the effect of extrapolating the orthorhombic fit to high angle is required.

Unstable Refinements 6 Students should discuss the use of restraints and constraints here. Particularly relevant would be the use of appropriate geometry restraints for the tetrahedral sulphate anion.

X-rays versus Neutrons? 6 Marks will be given for commenting on the differences that result from the high value of f for Pb for X-rays. Comments may also be made on polarisation and possible preferred orientation effects with Bragg-Brentano geometry.

(e) CIF Submission 5 Marks to be deducted for:

Lack of plain text (text file or PRE formatting required)
Excessive number of CIF output files (e.g. for wrong structure)
Errors in CIF file (unlikely unless CIF file is edited or created by hand)

	Category	Marks	General Comments on Mark Allocation
(a)	General Presentation	5	Marks are given for overall appearance, e.g. clean layout with clear headings, tables and pictures in appropriate places, and general attractiveness of the report.
	HTML & Navigation	5	Marks are given for ease of navigation (which is easier for single page documents), correct HTML links, and HTML that displays correctly. Marks are deducted if the course organiser has to alter the report so that it displays correctly on the screen. Marks will be deducted for non-generic HTML.
	Introduction	5	Report should be identified by author's name. The introduction should set the scene correctly. The student should demonstrate that he or she understands the aims and objectives of the exercise correctly, especially the concept of using subgroups to try lower symmetries.
(b)	Data Set A: Strategy	5	Marks are given for a description of the various steps taken. There is no need for a blow-by-blow account, which may be penalised. Marks will be deducted for incorrect strategies including refinement of anisotropic displacement factors, individual isotropic B values, or too many parameters in general.
	Tables	10	Full marks will be given for supplying appropriate tables and then marks will be deducted for: Missing captions Missing units Missing esd (su) values Missing R factors Missing sections of tables Wrong parameter or esd values Excessive parameter or esd values (i.e. too many digits) Excessive number of tables (start and end tables are sufficient)
	Figures	5	A figure should be given showing observed, calculated, and difference profiles together with calculated reflection positions. Marks will be lost for missing figure captions, excessive number (or inapprpriate use) of figures. Completely missing figures will result in zero marks. Figures may be included to show graphical background used. Best fit should show acceptable difference plot.
(c-i)	Data Set B: Strategy (for correct structure)	5	Marks are given for refining the low temperature structure with the correct space group P2(1)/n as demonstrated by achieving a correct value for beta (90.25° or 89.75°). Marks will be deducted for use of anisotropic displacement factors. Individual isotropic B values should be used.
	Tables	10	Full marks will be given for supplying appropriate tables and then marks will be deducted for: Missing captions Missing units Missing esd (su) values Missing R factors Missing sections of tables Wrong parameter or esd values Excessive parameter or esd values (i.e. too many digits) Excessive number of tables (start and end tables are sufficient) Wrong site occupancies Badly split atom in going from O(3) to new O(4)
	Figures	5	A figure should be given showing observed, calculated, and difference profiles together with calculated reflection positions. Marks will be lost for missing figure captions, excessive number (or inapprpriate use) of figures. Completely missing figures will result in zero marks. Figures may be included to show graphical background used. Best fit should show acceptable difference plot.
(c-ii)	Data Set B: Strategy (for wrong structure)	5	A very clear explanation of the strategy is required here given that students may either refine in P2(1)/b11 with x unique or P12(1)/a1 with y unique, but with the x and y coordinates swopped and a and b values swopped. No further marks to be deducted for similar errors made in previous refinements.
	Table & Figure	3 + 2	Marks are given for supplying appropriate table and figure of best fit using wrong space group. No further marks to be deducted for similar errors made in previous tables.
(d)	General Conclusions	6	Marks are to be given for identifying (as opposed to obtaining) the correct solution. The poorer fit relative to that at 300K could be commented on.
	300K versus 5K	6	Students should mention the following to achieve full marks: changes in lattice parameters, thermal parameters, and the monoclinic distortion that results.
	Data Set A versus B	6	The key point here is the difference between low- and high-resolution (in terms of d-spacing) data sets and the consequences of this, in particular the restricted use of parameters (i.e. a limited number) when the data set is limited. In addition, the fact that low resolution structures cannot distinguish between orthorhombic symmetry and a monoclinic distortion. The figure showing the effect of extrapolating the orthorhombic fit to high angle is required.
	Unstable Refinements	6	Students should discuss the use of restraints and constraints here. Particularly relevant would be the use of appropriate geometry restraints for the tetrahedral sulphate anion.
	X-rays versus Neutrons?	6	Marks will be given for commenting on the differences that result from the high value of f for Pb for X-rays. Comments may also be made on polarisation and possible preferred orientation effects with Bragg-Brentano geometry.
(e)	CIF Submission	5	Marks to be deducted for: Lack of plain text (text file or PRE formatting required) Excessive number of CIF output files (e.g. for wrong structure) Errors in CIF file (unlikely unless CIF file is edited or created by hand)

Note: this is a new marking scheme compared to previous years. In particular, the marks are enhanced for the conclusions so as to help grade the projects between poor, satisfactory, good, and excellent categories.

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