How can I perform calculations including the relativistic approximation SC-ZORA and solvent approximation COSMO in Gaussian 09?
Gaussian can’t effectively treat relativistic effects (look up integral=dkh [1] and try its limited use. AFAIK you can only have 50 parameters in internal coordinates which is not very realistic for common molecules today.) and has no ZORA implemented. It is also not able to do COSMO PCM calculations. Instead one would use other PCM schemes, e.g. IEFPCM which is the default in Gaussian.

So can you perform your calculations with Gaussian 09? Unfortunately not.

Instead, you should change to other programs that fit your needs better. As it is free for academic use [2], you can have a look at ORCA and if it fits to your needs.



[1] http://gaussian.com/integral/
[2] read the licence carefully, it is quite strict