Dialog Box: Interactions - Manual [Forces menu]
This dialog is for specifying interactions involving tau (torsion) or psi (bond-plane) angles, or involving bonds or 3-atom angles which do not share atoms. The index number of each individual generated coordinate (bond or atom) must be known, which can only be done by carrying out a preliminary calculation and obtaining the numbers from the printout. Again, this option specifies interactions between individual specific generated bonds and angles (including tau and psi), rather than types, as in the Interactions - Automatic option. Of course if the specifications of the primary force constants are changed so that the numbers change, the Manual Interactions may be invalid.
For interactions involving bonds and 3-atoms angles which share atoms, it is usually easier to use the Interactions - Automatic option.
The 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 lower list gives the specifications for each force constant in terms of pairs of primary coordinates - 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.
Specifications (lower list). In the fields labeled Coor1 and Coor2 you must enter the numbers of two generated coordinates (bonds and angles) - not coordinate types or fcons as listed in the dialogs of the Forces menu. The generated coordinate numbers must be obtained from the printout of a preliminary calculation - these numbers are in the sections below "Internal Coordinates Generated".
Note : there is currently a limit of 50 coordinate pairs per force constant.
For examples of this type of interaction specification, see the sample files BENZ.VBR and NI(CO)4.VBR
Certain types of interactions may be questionable because of the arbitrary direction and/or sign of the displacements. This includes: 1) out-of-plane coordinates for non-180-degree 3-atom angles; 2) either in-plane or out-of-plane coordinates for 180-degree 3-atom angles; and/or 3) psi angles for planar configurations (psi angle = 0). Of course this may depend on the particular geometry and symmetry.
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.