MacroModel Reference Manual - ISP

Chapter 4: Operation CodesThe constraint potential, E c , for a fixed bond angle is calculated using the equationE c= k( θ–θ 0) 2 ⁄ 2where θ is the current bond angle, θ 0 is the desired bond angle (arg6), and k is the forceconstant (arg5).If two FXBA commands specify the same bond angle, the second replaces the first.Note that for MBAE calculations, FXBA commands should be placed in a .sbc file.arg1 Atom 10 Clear all existing FXBA constraintsarg2 Atom 2arg3 Atom 3arg5 Force constant (default = 100 kJ/mol·rad 2 )arg6Desired nonzero angle (degrees)0 Use initial value.>0 Use this value.arg7Half-width of flat bottom well (degrees).FXTA — FiX Torsion AnglesThis command is used for restraining dihedral angles defined by the position of quartets ofatoms. Such wells, particularly when the flat-bottomed option is used, are useful when minimizingcrude structures in which some approximate torsional angle is known (e.g., from NMRcoupling constants) but an exact value is not available. They are also useful in moleculardynamics to prevent major conformational changes without affecting the local conformationaltrajectory of the simulation. Using arg7=0., the same simple restraint in MacroModel to fix atorsion angle at arg6 is used.If two FXTA commands specify the same torsion angle, the second replaces the first, unlessarg8 is nonzero.Note that for MBAE calculations, FXTA commands should be placed in a .sbc file.MacroModel 9.7 Reference Manual 71