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

This is for locating sets of tau angles using individual generated atoms rather than overall specifications based on type (atomic number) and length limits. For "automatic" tau specification of the latter type, see Tau (Torsion) Angles. The manual type of tau angle is not applicable to crystals or polymers.

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).

Each entry in the upper list is an "fcon" or force constant - this pertains to a set of actual tau angles, specified by the list of quadruplets of atom numbers in the lower list. This set may or may not be equivalent by symmetry, and whether or not a set of tau angles conforms with symmetry is up to you, when you specify the individual atom quadruplets (automatic tau angles specified in the Tau (Torsion) Angles dialog always conform with symmetry). All members of a symmetry-equivalent set should be included in one manual tau angle in this list, but one manual tau angle is not limited to a single symmetry-equivalent set - there may be several chemically similar angles included.

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.

The Atom numbers in the specifications in the lower list or spreadsheet must be the numbers of actual generated atoms in the molecule. These are best obtained from the Atoms Window - in fact you can select bonds involving atoms which have been clicked in succession with the mouse. This process, after a query (), will bring up this dialog. Generated atom numbers with coordinates can also obtained from the Output Window. The numbers entered here are not necessarily the Input Atoms as entered in the Input menu, although the lists of input and generated atoms will be the same if there is no symmetry. If there is symmetry, there will be more generated atoms than input atoms.

The sequence of atom numbers may be reversed - that is 1-2-3-4 is equivalent to 4-3-2-1.

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.