There is an undocumented input format change for the opt=modredundant calculation in Gaussian 09 revision C.01. For earlier versions, you could specify a value for a structure parameter (bond length, bond angle or dihedral angle) different from the value in the geometry specification. For example, the following input:

#p opt=ModRedundant B3LYP/6-31G

{molecule specification}

B 1 2 1.5000 F

would perform a geometry optimization calculation with a fixed bond length as 1.5 angstrom between atom 1 and 2 no matter what is distance between atom 1 and 2 in geometry specification.

However, this style input does not work in revision C.01. The modredundant optimization input format in Gaussian 09 revision C.01 is "B 1 2 F". The program will perform an optimization and fix the bond length between atom 1 and 2 to the value it has in the geometry specification. So if you want to optimize the structure with a fixed bond length of 1.5 angstrom, you have to modify the atomic coordinates to a structure with a 1.5 angstrom distance between atom 1 and 2. The format for the same calculation as above is:

#p opt=ModRedundant B3LYP/6-31G

{molecule specification, use GaussView to set bond length between atom 1 and 1 as 1.5 angstrom.}

B 1 2 F


For NMR calculations that use the CSGT method, the following keyword needs to be added to avoid getting an error message:

NMR=CSGT IOp(10/93=1)

This can be found on the Gaussian website
This method might also require using a larger basis set for accurate results. The GIAO method does not require the addition IOp and seems to generate the same results.