This is the main control dialog for SYMGRP. It serves two functions; to set some overall parameters. and to show the list of symmetry operators and allow addition, revision or deletion.
To generate a Cartesian symmetry matrix file (.SYM), first revise the overall parameters as described below; fill in the list of operators using the Add or Revise buttons, which call up the Symmetry Operator dialog; then click on the Calculate button. If the calculation is successful, you will be presented with a standard Save File dialog box, allowing you to save the .SYM file in the desired location
Note that you can save and re-read files (.msy) containing the overall parameters and the symmetry operators using the Save, Save As and Open commands in the File menu.
Overall parameters.
The Symbol is for your reference only - it is not used in the calculation.
If you check Centric, rather than Acentric, SYMGRP will add a code in the output .SYM file so that ATOMS or SHAPE can double the number of matrices and generate a center of inversion. Generally, to save disk space, you should not add operators in SYMGRP to the point that a center is generated by the operators themselves. If a center is generated, a message box will inform you of this.
Tolerance. When SYMGRP multiplies symmetry matrices together to generate new operations, it must have a tolerance to decide when a newly-generated matrix is identical to an existing one. The default tolerance, 0.000002 , is appropriate for 4-byte floating-point numbers and should be used whenever the orientations of the symmetry elements are known. Never use a tolerance smaller than this. If you are finding the orientations of symmetry operators by trial and error, you may need to set the tolerance to a larger value, decreasing it in successive iterations as the orientation is refined.
If you know the exact orientation of the symmetry operators, the Limit to matrices, that is the number of matrices generated, should be at least as large as the (known) number of matrices or operations. If you do not know the exact orientation of some of the symmetry elements; you will know that your guess for the operator is wrong, or your tolerance is too small, if too many operations are generated. The ideal number is somewhat larger than the number of known operators in the group.
The identity operation is provided automatically and does not need to be entered.
Rotate all. This group of controls allows rotation of all symmetry operations by the specified number of degrees about the specified Cartesian axis. Thus any orientation may be attained.