Jul 6, 2011 3:35:40 PM (10 years ago)

update documentation

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  • branches/sandbox/doc/expgui6.html

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    2323<TR><TH><A  Href="expgui.html">EXPGUI top</A>
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    34 <h3>A.6 Constraints panel</h3>
     34<h3>A.6 Re/Constraints panel</h3>
    3636GSAS offers two types of restraints that can be included in a model.
    4141by grouping parameters so that a single shift (possibly scaled) is
    4242applied to each parameter in the group.
    43 The constraints panel is used to set these "hard constraints."
     43The Re/Constraints panel is used to set these "hard constraints" or in
     44  the case of bond distances, "restraints".
    4445At this time the only constraints that can be set in EXPGUI are on atomic
    45 and profile profile (others can be set in EXPEDT).
     46and profile profile (others can be set in EXPEDT). EXPEDT offers many
     47  other types of restraints as well.
    4749Note that these constraints are only applied to shifts, but not to the
    212214usually make sense and EXPGUI does not allow it.
     216<a name="DistanceRestraints"></a>
     217<H4>Distance Restraints</H4>
     219  When the distance restraints tab is first selected, the active
     220  distance restraints for the selected phase are shown. The <B>Atom 1</B> and
     221  <B>Atom 2</B> buttons can be used to sort this list.
     222  <P>
     223  The <B>Restraint Weight</B> parameter is very important. As this
     224  determines how strongly the distance restraints will be used in the
     225  refinement relative to the diffraction data. This value is sometimes
     226  set as high as 500,000 in the initial stages of a fit, but should be
     227  lowered as much as is possible in the final stages of the fit, as
     228  needed to obtain chemically plausible interatomic distances. For
     229  some refinements, a value of 0.0 is possible for the Restraint
     230  Weight in the final stages. This removes any restraint effect.
     232  To see how restraints are affecting the refinement, one can look at
     233  the ratio of the "Sum(w*d**2)" (shown in the .LST file) for the
     234  diffraction histograms vs. the restraint histogram.
     236  The <B>Active Restraints Editor</B> can be used to select distances
     237  to be restrained or edit the restraints. If there are too many
     238  distances shown, the options at the top of the window can be used to
     239  select a distance range, or specific atoms to include. Likewise,
     240  only restrained or unrestrained distances can be selected. The
     241  changes are applied when the <B>Filter</B> button is pressed.
     243  The bottom section of the window shows individual interatomic
     244  distances. A <B>Restraint</B> and <B>Tolerance</B> value is needed to restrain
     245  that distance. The quantity minimized is
     248 "Restraint Weight" * SUM{  [ (Distance - Restraint) / Tolerance ]**2 }
     251thus the "Tolerance" provides individual weighting for each
     252  restraint.
     254  To set a large number of restraints at the same time, select the
     255  check button next to the distance(s), or use the <B>Check
     256  All</B>. Then use the section above to enter a global
     257  <B>Restraint</B> and <B>Tolerance</B> value. These values are set
     258  when the <B>Set Checked</B> button is used.  The <B>Delete
     259  Checked</B> button removes restraints from all selected distances.
     261The entered restraint information will be used onlt when the <B>Save
     262  Changes</B> button is pressed. The <B>Cancel</B> button will reverse
     263  any changes made while the window is open. Closing the window has
     264  the same effect as "Cancel."
     265  <P>
     266  Note that the interatomic distances that are shown are determined by
     267  the parameters to program DISAGL. These are controlled with the
     268  <B>Edit Search Distance Parameters</B> button.
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