[Fe(Cl)4]3- is a tetrahedral complex, so building it's topology for charmm/gromos/amber force fields is trivial. Getting a correct set of parameters for bonds, angles and dihedrals is, however, not that easy and requires QM calculations of the species (it's a ralatively small system, so such calculations with vibrations shou;d take at most 1-2 days). As sugested above, you should start with something like CGenFF.

You can generate ipt file for GROMOS force field visiting the following web link: http://compbio.biosci.uq.edu.au/atb/index.py?tab=home_tab

Dear Uttam, Your compound is much difficult to parametrize. You need a lots of QM calculation. Please visit CHARMM CGenFF page.

http://dogmans.umaryland.edu/~kenno/cgenff/download.shtml

[Fe(Cl)4]3- is a tetrahedral complex, so building it's topology for charmm/gromos/amber force fields is trivial. Getting a correct set of parameters for bonds, angles and dihedrals is, however, not that easy and requires QM calculations of the species (it's a ralatively small system, so such calculations with vibrations shou;d take at most 1-2 days). As sugested above, you should start with something like CGenFF.

Thanks Bartosz. Yes, the complex is tetrahedral. I also know the Fe-Cl distance (2.189 Angstrom) from the literature. I have installed GAMESS and I am trying to optimize the geometry and obtain the atomic partial charges.  I need to provide the net charge and multiplicity of the complex for QM calculations.  Its net charge is  -1.  I don't know its multiplicity. I am from physics background. Can you tell me how I can obtain its multiplicity? Also which method (RHF  or B3LYP etc.) is suitable? 

http://en.wikipedia.org/wiki/Multiplicity_%28chemistry%29

CGenFF and most biomolecular force fields aren't going to be able to handle this type of complex. You'll have to parametrize it de novo based on QM target data.  That includes bonded and nonbonded (charge and van der Waals) terms.  A purely additive model here is likely to suffer many defects but you can probably get something reasonably close.

It seems to me that the task is not so difficult as it might seem at first glance.

Biochemical force fields do not require very careful parametrization of the valence parameters (as opposed to nonvalent). Quantum-mechanical calculations are inevitable. However, this is not an additional difficulty since they anyway needed to determine partial charges. I would use the minimum bases for this like:

Def2-svp for the energy/geometry

Def2-svpd for charges

For example:

Draw a molecule in a graphics editor and pre-optimize using molecular mechanics. I see a tetrahedron. The bond length is 2.27 A. Not bad for molecular mechanics.

Regarding multiplicity. We should expect a few unpaired electrons from the iron. Try 3, 5 and 7 open shells: -3103.18922179871, -3103.24599208681 and -3102.98425514332 Hartree at pbe0-D3/Def2-svp. The minimum energy is at 5 (+1 = 6 multiplicity).

Optimize geometry in pbe0-D3/Def2-svp. Tetrahedron with Fe-Cl 2.22 A.

Determine the partial charges in pbe0/Def2-svpd: Cl -0.465, Fe 0.86.

Force constants for bonds and angles can be derived from quantum calculation. It is enough to calculate a few points with deformations of bond and angle.

Van der Waals parameters of chlorine can be obtained from AMBER, or any other FF. For the iron they can be calculated in the same pbe0-D3/Def2-svpd using probe atom, say argon. The last two steps I did not, because it takes about an hour. It is the longest throughout the procedure.