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Ab initio methods attempt to model the molecular orbitals (MOs), typically by making
assumptions regarding the form of the MOs. For example, since electronic motions are
much faster than nuclear motions the two are considered to be decoupled (Born-
Oppenheimer approximation). Since the Schrödinger Equation is difficult to solve
simultaneously for two (or more) electrons, the Independent Electron Model is often
used, under which each electron is taken to move under the influence of the averaged
interaction of the other electron. Lastly, the Linear Combination of Atomic Orbitals
(LCAO) approximation is common; under LCAO the MO is assumed to be a linear
superposition of the electronic wave functions of the participating atoms.[113] ,
Experimental data used in parameterizing force fields includes infrared and microwave
absorption spectra of vapors of small molecules, attributed to normal mode motions of
the molecules[114,115]. Equilibrium bond lengths and angles, as well as van der Waals
radii, can be obtained from crystallographically obtained structures. Jorgensen et al
famously made initial guesses for the van der Waals parameters and then varied them
until simulations of pure liquids reproduced experimentally determined values of
thermodynamic quantities such as density and latent heat of vaporization[116].
The most accurate and widely used force field based simulation method is Molecular
Dynamics (MD). MD codes solve Newton’s equations of motion for point particles
under such a force field. Many stochastic methods, collectively referred to as Monte
Carlo methods, exist and are widely used for energy minimization and conformational
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