Tau (Torsion) Angle Force List
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Dialog Box: Tau (Torsion) Angle Force List [Forces menu]

This is for "automatic" torsion angle location - VIBRATZ searches the structure for torsion angles using the specified atom types (usually atomic numbers) and distance and angle limits. For "manual" torsion angle location, using sequence numbers of specific generated atoms, or by point-and-click, see Manual Tau.

The upper list gives the current 4-atom tau angle forces. The data for each one is in a single line in the scrolling list or spreadsheet. To add a new entry, press the down arrow key while the focus is on the last row, or click the Add button. The lower list gives the specifications for each force constant in terms of atom types - an unlimited number of specifications is allowed (there must be at least one).

No. This is the number in the angle list.

Fcon This is the number in the overall force constant list.

Force. Note that angle forces may be specified in either in md-A or in md/A. This choice is made in the Control window or the Basic Parameters dialog (Settings menu).

Nspc. Number of atom type specifications for this angle (lower list).

Lst Sq. If this box is checked, this force constant will be included in least-squares refinement.

Dmax12/Dmax23/Dmax34. These are the maximum bond distances for each of the three legs. If a value is zero it will be reset to the default maximum ( Basic Parameters dialog).

Dmin23. This is the mimimum distance for the middle leg. The two middle atoms in a torsion angle need not be connected to each other with an actual chemical bond. In the example ALLENE the second and third atoms are taken to be the two end atoms of a linear C-C-C configuration, while the central C atom is ignored. DMin23 is required to exclude the two shorter C-C bonds.

TauMin/Taumax. The angle limits are applied to the absolute value of the tau angle. Tau angles assume the sign convention as defined by WDC, p. 60; the angle is positive if, viewing the atoms along the bond 2-3 with 2 nearer the observer than 3, the angle from the projection 2-1 to the projection of 3-4 is traced in the clockwise sense. The dog-leg configuration in the diagram in the dialog shows a tau angle near 180; an angle near 0 would give a U-shaped pattern. Thus using the absolute limits assumes that it is more important to distinguish between dog-leg and U configurations than between left- and right-handed intermediate configurations. Angle limit values of zero will skip angle tests.

The atom types in the lower specification list should key to entries in the Atom Types list - normally the atomic number is used. Types must be given for each of the four atoms involved in the sequence shown in the diagram to the right.

Once specifications have been entered, the force-constant values and least-squares flags may be changed in the overall Forces List, accessible from the Control Window.