In a Z-matrix for the Gaussian programs, letters can be used to specify some or all of the internal coordinates. For example, a Z-matrix for HOCl can be written as
O1
H2 1 R1
Cl3 1 R2 2 A1
Variables:
R1 0.96
R2 1.6
A1 108.0
If one wanted to hold the bond angle constant at 108o during a geometry optimization, the last line of the preceding Z-matrix would be replaced by
Constants:
A1 108.0
Do include a space before each of the words Variables and Constants. (In Gaussian 98 and 03, the keyword Opt must be replaced by Opt=Z-matrix.) Set up a Z-matrix for a partial geometry optimization for C2H6 in which the dihedral angles are held fixed at values corresponding to (a) the staggered form; (b) the eclipsed form. Then perform HF/6-31G** optimizations on these two conformations and take the energy difference to find the predicted barrier to internal rotation in kcal/mol.