These instructions are used to direct the placement of the origin point ORGN (specified by instruction 501, 505, or 506) onto the drawing (dimensioned by instruction 301). In addition the three-dimensional assembly of atoms (chosen by the 400 series instructions) constituting the model is scaled. The atomic centers of the model will then be hanging in space above and below the drawing board, correctly positioned to be projected from the eye point described with instruction 301.

• 60x: Normal Modes of Positioning and Scaling
• 61x: Incremental Modes of Positioning and Scaling

Several normal modes of operation are available to the user for positioning and scaling the model. Instruction 601 requires the user to supply a complete explicit description of position (X0,Y0) and scale (SCAL1). At the other extreme, instruction 604 automatically scales and positions the model so that the peripheral projected atom centers will touch two opposite borders and the peripheral atoms in the remaining dimensions will be centered on the drawing. An intermediate mode is available through 602, which provides automatic scaling after explicit positioning. This usually allows only one edge of the model to touch a border. Finally, instruction 603 requires an explicit scale and does automatic centering.

In general, the 604 instruction is the easiest and safest one to use, but situations arise in which the user should not relinquish control to the program. For example, if a big illustration is to be drawn piecewise on a small plotter, the user will have to maintain control over the scale, and probably over positioning, so that the partial plots can be fitted together correctly.

A second scale factor SCAL2 is required in connection with the ellipsoid (or sphere) size. It is a dimensionless scale factor ratio used to modify all rms displacement values before plotting equiprobability ellipsoids or spheres. SCAL2 values for given probabilities are tabulated in various books on statistics such as D. B. Owen's Handbook of Statistical Tables. The primer constant for SCAL2 is 1.54, which corresponds to 50% probability.

ORTEP-III saves the user the time of looking up the SCAL2 value corresponding to a desired probability. If the value entered in columns 37-45 is a negative whole number in the range -1 to -99, it is interpreted as the probability of the ellipsoids or spheres, and the value of SCAL2 is set by the program by table lookup.

If the instruction's entry for X0, Y0, SCAL1, or SCAL2 is "0" or blank, the primer constant value is used. This means that an X0 or Y0 cannot be entered as exactly zero. If zero is wanted, a small nonzero number should be entered.

Additional flexibility is provided through the incremental instructions 611, 612, and 613. These allow the user to "nudge" the model or modify the scale factor (SCAL1), or both, after the parameters have been initially set with a previous 600 series instruction. The 611 instruction adds X0, Y0 to the previous X0, Y0 position for the ORGN and multiplies the existing SCAL1 by K (except if K = 0, SCAL1 is unmodified). Instruction 612 increments the position and then does an automatic scaling; 613 first increments the scale (by multiplying by K) and then automatically repositions.

A conservative general approach is to follow a 604 with a 611 having X0 = 0, Y0 = 0 and K = 0.9. This will simply reduce the scale 10% about the origin so that there is more space for labels, etc.

Columns	Instruction 611	Instruction 612	Instruction 613
7-9	611	612	613
10-18	X0	X0	-
19-27	Y0	Y0	-
28-36	K	-	K
37-45	SCAL2 or probability	SCAL2 or probability	SCAL2 or probability

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Page last revised: February 19, 1996