Molecular Mechanics

[Paddy Kane <94970459 ( ( at ) ) vax1.dcu.ie>]

  Hi,
  I am a HyperChem user and I have two questions about molecular
 mechanics (MM):
  I recently asked the following question
 >  While I know that the energies obtained in an MM calculation have
 > no absolute  meaning, would the difference in energies of two
 > optimized conformations of the same complex have any meaning? If
 > so, what kind of energy are we talking about?
  What I wanted to know but did not specifically ask (typical of the
 way I ask questions!!!!!) is that if I have two different
 complexes where the ligand is the same but the ion is different what
 is the significance of the difference in energy resulting from an
 (MM) optimization?
  The reason that I ask is that I obtained the following energies from
 MM optimizations for complexes of the same ligand but with different
 ions:
 		Energy/ (kcal / mol)
 Li+		-190.6
 Na+		-171.2
 K+		-150.8
  We have found experimentally that the ligand is selective for
 sodium over magnesium; so I would have expected to have obtained a
 lower energy for the sodium complex. So *IF* MM optimization
 energies are not a reliable indicator of selectively what exactly is
 the significance of the above difference of 63 kcal / mol?
  The above optimizations were carried out by assigning the atom types
 to the metals published by Grootenhuis and Kollman, JACS., 1989, 111,
 2152-2158. However I am not aware of the existence of atom types in
 the case of Group 2 metals and so I propose to use Noble Gas atoms as
 substitutes with charges of 2+ but I am not certain as to what atom
 types to use. Should I use the Noble Gas atom types that are part of
 the MM+ force field or should I leave the atom type as **.
 Regards,
 Paddy
 Paddy Kane
 E.mail: 94970459 ( ( at ) ) vax1.dcu.ie
 School of Chemical Sciences
 Dublin City University
 Dublin
 Ireland