I have tried several functionals, but only one functional was able to optimize the desired structure. With the successful structure, I tried to optimize again by implementing ZORA or ECP on the metal but the geometry will be broken. I am using the ORCA program.
link
http://pubs.acs.org/doi/abs/10.1021/acs.jpca.5b10996?journalCode=jpcafh
http://www.unige.ch/sciences/chifi/publis/refs_pdf/ref01470.pdf
http://link.springer.com/article/10.1007/s00894-007-0259-7
http://dft.uci.edu/pubs/RCFB08.pdf
http://www.yorku.ca/renef/pra-4dtm13.pdf
Using ECP may sometimes lead to completely wrong geometries, particularly for systes where the metai ions is bonded to electron-rich elements. This is because there isn't simply enough electrons from the metal side to properly describe such interaction. But also make sure that you're specyfying the ECPs in a correct way.
Different DFT functionals may obviously give you different geometries, but the differences are usually not that large. There may be, however, instances where for particular systems some functionals do not converge well. I would say that this is rather rare, though and it may indicate that there's something wrong with either the input files or input geometries.
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