There are several ways to do partial optimization (freeze coordinates) in Gaussian.
- One of them is to add “ModRedundant” to Opt or Geom keyword and to write
F1 F2 F3 ...
after coordinate section. “F” here means freeze, and integer stands for atom number.
- Another one is to add “ReadOpt” to Opt keyword and to add
after Cartesians. This will exclude first four atoms from optimization. BTW, you definitely should read the section about “ReadOptimize” option if you haven’t before.
- The third one is using a freeze-code. I came across this method in the post at ResearchGate portal. The idea is simple: you add 0 (or nothing) between atom symbol and its coordinates if position of the atom should be optimized, and -1 if you want to freeze it.
C 0 1.5841970000 -2.9056620000 0.0000300000 C -1 3.2294750000 0.7041530000 0.0001550000
The first atom in the example will be included in the optimization and the second will be frozen.
Just that simple!
And something more: if you restart your optimization from .chk file, freezing will be preserved. Enjoy!