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occurs because the value of the energy gradient near<br />

a saddle point, as near a minimum, is very small. For<br />

example, at point (c), the derivative of the energy is<br />

0, and as far as the minimizer is concerned, point (c)<br />

is a minimum. First derivative minimizers cannot,<br />

as a rule, surmount saddle points to reach another<br />

minimum.<br />

NOTE: If the saddle point is the extremum of interest, it is<br />

best to use a procedure that specifically locates a transition<br />

state, such as the CS MOPAC Pro Optimize To<br />

Transition State command.<br />

You can take the following steps to ensure that a<br />

minimization has not resulted in a saddle point.<br />

• The geometry can be altered slightly and<br />

another minimization performed. The new<br />

starting geometry might result in either (a), or<br />

(f) in a case where the original one led to (c).<br />

• The Dihedral Driver can be employed to<br />

search the conformational space of the model.<br />

For more information, see “Tutorial 5:<br />

Mapping Conformations with the Dihedral<br />

Driver” on page 38.<br />

• A molecular dynamics simulation can be run,<br />

which will allow small potential energy barriers<br />

to be crossed. After completing the molecular<br />

dynamics simulation, individual geometries can<br />

then be minimized and analyzed. For more<br />

information see Appendix 9: “MM2 and MM3<br />

<strong>Com</strong>putations”<br />

You can calculate the following properties with the<br />

computational methods available through Chem3D<br />

using the PES:<br />

• Steric energy<br />

• Heat of formation<br />

• Dipole moment<br />

• Charge density<br />

• COSMO solvation in water<br />

• Electrostatic potential<br />

• Electron spin density<br />

• Hyperfine coupling constants<br />

• Atomic charges<br />

• Polarizability<br />

• Others, such as IR vibrational frequencies<br />

Molecular Mechanics<br />

Theory in Brief<br />

Molecular mechanics describes the energy of a<br />

molecule in terms of a set of classically derived<br />

potential energy functions. The potential energy<br />

functions and the parameters used for their<br />

evaluation are known as a “force-field”.<br />

Molecular mechanical methods are based on the<br />

following principles:<br />

• Nuclei and electrons are lumped together and<br />

treated as unified atom-like particles.<br />

• Atom-like particles are typically treated as<br />

spheres.<br />

• Bonds between particles are viewed as<br />

harmonic oscillators.<br />

• Non-bonded interactions between these<br />

particles are treated using potential functions<br />

derived using classical mechanics.<br />

• Individual potential functions are used to<br />

describe the different interactions: bond<br />

stretching, angle bending, and torsional (bond<br />

twisting) energies, and through-space<br />

(non-bonded) interactions.<br />

• Potential energy functions rely on empirically<br />

derived parameters (force constants,<br />

equilibrium values) that describe the<br />

interactions between sets of atoms.<br />

• The sum of interactions determine the spatial<br />

distribution (conformation) of atom-like<br />

particles.<br />

<strong>ChemOffice</strong> 2005/Chem3D <strong>Com</strong>putation Concepts • 131<br />

Molecular Mechanics Theory in Brief

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