Constrained optimization refers to the optimization of molecular structures (transition state or minimum-energy) in which certain parameters such as bond lengths, bond angles or dihedral angles are fixed. Jon Baker’s Optimize package, implemented in Q-Chem, makes it possible to handle constraints directly in delocalized internal coordinates using the method of Lagrange multipliers (see Appendix A). Features of constrained optimization in Q-Chem are:
Starting geometries need not satisfy the requested constraints.
Constrained optimization is performed in delocalized internal coordinates, which is typically the most efficient coordinate system for optimization of large molecules.
Q-Chem’s free-format $opt section allows the user to apply constraints with ease.
Constraints are imposed via the $opt input section, whose format is shown below, and the various parts of this input section are described below.
Note: As with the rest of the Q-Chem input file, the $opt section is case-insensitive, but there should be no blank space at the beginning of a line.
$opt CONSTRAINT stre atom1 atom2 value ... bend atom1 atom2 atom3 value ... outp atom1 atom2 atom3 atom4 value ... tors atom1 atom2 atom3 atom4 value ... linc atom1 atom2 atom3 atom4 value ... linp atom1 atom2 atom3 atom4 value ... ENDCONSTRAINT FIXED atom coordinate_reference ... ENDFIXED DUMMY idum type list_length defining_list ... ENDDUMMY CONNECT atom list_length list ... ENDCONNECT $end