164 COMPUTER HARDWARE, SOFTWARE
fi eld calculations or just force fi eld calculations. Because force fi eld parameters
are dependent on the potential energy functions, the entire set of functions
and parameters are often referred to as “ the force fi eld. ” In essence, the
molecular mechanics method interpolates the structure and strain energy of
an unknown molecule from a series of similar molecules with known structure
and properties. As mentioned above, molecular modeling of transition metal
compounds is complicated by the partially fi lled d orbitals of the metal ions.
However, the structure of a coordination compound, and thus its thermody-
namics, reactivity, and electronic behavior, is strongly infl uenced by the ligand
structure. Because empirical force fi eld calculations have been very successful
in modeling organic ligands, it follows that these techniques should be extend-
able to transition metal coordination complexes as well. The problem remains
thatmolecular mechanics interpolates the structure of an unknown inorganic
complex from a set of parameters derived from fi tting a number of X - ray
crystallographic structures. If the complex being investigated differs signifi -
cantly from the structures used to determine the parameters, poor results will
be obtained.
Specifi c molecular mechanics (MM) force fi elds have been assembled, and
several of these are mentioned here. The MM2 molecular mechanics force fi eld
and its applications to inorganic chemistry are described in reference 5a,b and
references therein.^5 The MM3 force fi eld, a third - generation refi nement, is one
used extensively in molecular mechanical computations.^6 For applications in
inorganic chemistry see reference 7. Information on the MM2 and MM3 force
fi elds is available on the website http://europa.chem.uga.edu/. The MMFF94
Merck molecular force fi eld is described on the website http://server.ccl.net/
cca/data/MMFF94/ and in reference 8. MMFF94 achieves MM3 - like accuracy
for small molecules, yet is as applicable to proteins and other systems of bio-
logical signifi cance. The MMX force fi eld^9 has been used in many inorganic
applications, and one student exercise using MMX implemented in PCMOD-
EL TM (Serena Software, Inc.; see Section 4.5) is described in reference 10.
Many other force fi elds have been described. A few of these are as follows:
MOLBLD^11 ; MOLMECH, a molecular mechanics program containing an
extended MM2 force fi eld^12 ; and SHAPES.^13 UFF, a molecular mechanics
generic force fi eld has been used in inorganic chemistry applications.^14
MOMEC97 is a molecular mechanics program adapted to HyperChem TM as
described below in Section 4.5. More information is available at Professor
Comba ’ s website http://www.uni - heidelberg.de/institute/fak12/AC/comba/.
The current MOMEC force fi eld is described on this website.
The reference 4 authors discuss criteria that should be applied when describ-
ing molecules with these molecular mechanics programs. Some of these are as
follows: (1) Check the error fi le for interactions not in the parameter set,
because some programs will assign a force constant of zero to unrecognized
atom types; (2) check all interactions generating > 5 kJ/mol of strain to deter-
mine, for instance, whether that bond or angle really is that strained or whether
there is a parameterization or molecular structure problem; and (3) check the