Partial optimization in Gaussian09

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

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
noatoms atoms=1-4

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!