Thank you very much, Dr Oleg. I will do more reading based on your suggestion and see how it works.

Thank you very much!

You might like to try the ZORA approach in ORCA, they have some efficient basis sets available for this method.

Do you only need scalar relativistic effects or do you need to include spin-orbit coupling? Most DFT codes will allow you to perform scalar relativistic calculations using either DKH, ZORA or X2C Hamiltonians, but less can handle spin-orbit coupling correctly. The TURBOMOLE code allows two-component DFT as, I believe, does ADF. DIRAC allows full four-component calculations. MOLCAS can perform spin-orbit coupled CASSCF calculations, although the spin-orbit coupling is not  introduced until after the optimisation of the SOC-free wavefunction. I'm sure that other codes have these functionalities as well.

Best wishes,

Andy

Thank you for your attention.

I am trying to address the problem of metal-metal interaction, especially Au-Au interaction.

Many gold complexes are found to have intermolecular or some time intramolecular Au-Au distance of 3.0-3.5 angstrom. It is explained that due to relativistic effect and orbital constraction, the d-filled orbital can interact with p-empty orbital, and the Au-Au interaction reduces the overall energy of the system.

I am trying to simulate that interaction to find optimized geometries and also their electronic properties, such as frontier orbital energies; electronic transition...

Thank you!

If you are looking at the effects of orbital contraction, then a scalar relativistic model should be sufficient, in which case you can look at the optimised geometries and molecular orbital energies from Gaussian calculations in which you use the DKH Hamiltonian and compare to the same calculations when you use the standard, non-relativistic Hamiltonian. The DKH Hamiltonian should model the orbital contractions well enough, although a full SOC-calculation would reveal further subtleties.

Best wishes,

Andy 

Thank you very much for your suggestion, Dr Andrew. I will try first to see how it responds.

When using Gaussian for relativistic calculations, what type of basis set should I specify? (The Gaussian website does not specify any of the basis sets listed asa explicityly suitable for relativistic calculations).