Geometry Optimisation with CASTEP

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2 O j 2 O j 2 O j %ENDBLOCK IONIC_CONSTRAINTS For two constraints, one involving two sulfur ions, and another involving three oxygen ions. All coefficients not specified are assumed to be zero. The first column in the block gives a unique number to the constraint specified. The second specifies the species by either atomic symbol (S or O in the above example) or atomic number. The third column is the index <strong>with</strong>in a species current constraint are then specified. . The co-efficients of the three spatial co-ordinates for this ion under the As an example, let us consider the case of restricting a single ion to move along along a plane parallel to . The normal to this plane is and our constraint is that the dot product of this normal <strong>with</strong> the position vector of the ion in question is zero. If this is the second ion in the fourth species then In this trivial example, we can see that we need (if the fourth species is say, sulfur) %BLOCK IONIC_CONSTRAINTS 1 S 2 -1 1 0 %ENDBLOCK IONIC_CONSTRAINTS to satisfy the above equation. A variety of constraints can be specified in this fashion. For simplicity, a special cell file keyword is provided to fix the centre of mass in the calculation. fix_com = true
It is possible to fix the positions of all ions and hence only conduct dynamics for the cell degrees of freedom using the following. fix_all_ions = true Similarly, it is possible to specify that all cell degrees of freedom remain fixed. fix_all_cell = true Fixing both the cell and ionic degrees of freedom in this way will eliminate all motions and will hence return an error. Non-linear constraints, such as those used to fix the distance between two ions, are not yet supported. Constraints on individual cell degrees of freedom are also not yet supported. Next: Atomic masses Up: The .cell file and Previous: Constraints on the cell 2005-04-04
Page 1 and 2: Next: Recipe Geometry Optimisation
Page 3 and 4: Next: Inputs Up: Geometry Optimisat
Page 5 and 6: Next: The Size and Shape Up: Inputs
Page 7 and 8: In both cases if [units] are not sp
Page 9 and 10: After the atomic symbol or the atom
Page 11 and 12: KPOINTS_MP_SPACING [units] Where is
Page 13 and 14: 2005-04-04
Page 15: Next: Atomic masses Up: The .cell f
Page 19 and 20: Next: External pressure that is Up:
Page 21 and 22: Next: General Job Parameters Up: In
Page 23 and 24: When a calculation that was stopped
Page 25 and 26: Next: Electronic Minimisation Param
Page 27 and 28: Next: Example - Bulk Silicon Up: Th
Page 29 and 30: the default value is 3 but it must
Page 31 and 32: We must now choose a supercell to e
Page 33 and 34: Next: The .castep file Up: Geometry
Page 35 and 36: 4 -2.14844155E+002 -9.84322032E-003
Page 37 and 38: BFGS: Final Configuration: ========
Page 39 and 40: Next: Other output files Up: Output
Page 41 and 42: Next: A Variable Cell calculation U
Page 43 and 44: arbitrary set of basis set paramete
Page 45 and 46: 3 -2.16474423E+002 2.10512829E-007
Page 47 and 48: Figure 6:A finite vacuum gap may ca
Page 49 and 50: 0.0000000000 0.0000000000 24.503725
Page 51 and 52: thickness We can see that the total
Page 53 and 54: It is interesting to look at the ev
Page 55 and 56: Next: About this document ... Up: G

Geometry Optimisation with CASTEP

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