Interactions - Automatic
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Dialog Box: Interactions - Automatic [Forces menu]

This option will automatically locate interactions involving bonds and 3-atom valence angles sharing one or two atoms. It cannot be used for interactions involving tau or psi 4-atom angles, or bonds and 3-atom angles which do not share atoms. For these other types of interaction, use the Interactions - Manual option.

The left upper list gives the interaction 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 right upper 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 automatic interaction list.

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

Force. The force constants for interactions may be entered either in variable units (md/A for bond/bond; md for bond/angle and md-A for angle/angle interactions) or in constant md/A. The choice between the two types of force is made in the Control window or the Basic Parameters dialog in the Setting menu. See that dialog for the details of calculation using the two types of units, and formulas for interconversion of force constants.

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

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

Type. The type, or Sharing code specifies which atoms are shared, according to the diagrams below. The small dot denotes atoms which are shared. Note that the number of atoms shared, either one or two, distinguishes among several type of configuration, and that the code number increases as the sharing becomes more "remote" and presumably less important. The most common types of interactions have code zero (the default), and thus it is seldom necessary to actually specify this number.
bndang

AngMin/AngMax. The minimum and maximum angle limits apply in only two (common) cases - bond/bond interactions and angle/angle interactions with a shared leg (same central atom). The angle is between the non-shared legs. These limits may be needed, for example, in distinguishing different types of interaction in octahedra. They do not apply to angle/angle interactions with only the central atom shared. If the limits are zero there will be no testing of angle values.

Specifications (right upper list). The numbers of the two primary force constants or types of internal coordinates (bonds or 3-atom angles) must be entered in the Prim1 and Prim2 edit boxes at the left. The bonds or angles of the primary coordinates may share either one or two atoms, whose types must be given in the second two boxes (Com1 and Com2). When only one atom is shared, the second number must be zero. The types are those given in the Atom Types list, which are typically atomic numbers, not the sequence numbers of the Input Atoms.

The list box at the bottom of the dialog gives the fcon number, atom types and force constant for the current bond and 3-atom angle force constants. The Prim1 and Prim2 entries in the the interaction specifications must be taken from the fcon numbers, and the Com1 and Com2 entries must be taken from the corresponding atom types in this list.

Interactions involving 1) out-of-plane coordinates for non-180 degree angles; or 2) either in-plane or out-of-plane coordinates for 180 degree angles, may be questionable because of the arbitrary direction and/or sign of the displacements (see

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.