Any methods for your calculation are extremely sophisticated in terms of how methods work the best for the particular reaction you are looking at, and almost all of them are still at the laboratory level.
First thing that come to my mind is MM/QM hybrid methods. You'll probably need to consult someone of a laboratory dedicated to such calculations. Most of the time heavy calculation requires the use of supercomputers. Those supercomputers probably use optimized software for that, because software like Gaussian are meant for commercial computers.
I don't think any calculation with a freeware on a regular computer would converge if the reaction is complicated, much less will provide you with any reliable result.