Actually for the case of polyoxometalate i think the common theme Is Density Functional Theory (DFT) and wave function method

http://rel-qchem.sav.sk/

https://github.com/dftlibs/xcfun

http://tdrsp.soft112.com/

https://github.com/MRChemSoft/mrchem

http://www.cpmd.org/

http://www.freechemical.info/freeSoftware/OpenMX.html

http://www.freechemical.info/freeSoftware/Quantum_ESPRESSO.html

this list also provide database of software supporting DFT

https://en.wikipedia.org/wiki/List_of_quantum_chemistry_and_solid-state_physics_software

Mr. Brünle,

Using Gaussian (www.gaussian.com) and GaussView (http://gaussian.com/gv6features/).

Can you clarify your needs? e.g.: Is the POM in a salt or in solution, or in vacuum? Are biomolecules around? What properties are you interested in? Polyoxometalates are typically very heavy and can be challenging to model, even by DFT, if you go for "ab initio" modelling. But since you are in structural biology perhaps you rather work with some kind of force fields? Giving more details about the question would help people reply with more precise answers too.