Gaussian input with a constraint in an xyz optimization

To freeze a bond length, angle or dihedral, set the Opt=ModRedundant directive:

%nprocshared=20
%mem=40GB
%chk=/global/archive/phchm/ir-goettingen/s0opt2/ir_opt.chk
#P b3lyp/def2SVP opt=ModRedundant scf=(tight,MaxCycle=500) pop=full iop(6/7=3) gfinput gfoldprint gfprint EmpiricalDispersion=GD3 SCRF=(PCM,Solvent=Acetonitrile)

Ir complex with B3LYP

1 1
C -2.38008 0.32587 0.65976
S -4.86842 0.34670 -0.74159
S -5.07214 1.14093 0.59786
C -3.51219 -0.45064 -1.51185
...
Ir 0.59352 0.00001 -0.00005

D 1 2 3 4 0.00 B
D 1 2 3 4 F

D 1 2 3 4 0.00 B defines a dihedral (other options: X for a Cartesian coordinate, B for a bond length, A for an angle, L for a linear bend), the four atoms involved (numbered as in the xyz block), its value (0.00 here) and B for “build”, i.e. create this redundant coordinate.

The next line, D 1 2 3 4 F, freezes that dihedral at the value given above – here at 0 degrees.