Molecular alternate calculation method

The alternative simulation approaches are based on molecular dynamics calculations. This is conceptually simpler that the Monte Carlo method the equations of motion are solved for a system of A molecules, and periodic boundary conditions are again imposed. This method pennits both the equilibrium and transport properties of the system to be evaluated, essentially by numerically solvmg the equations of motion... 

For the carbonyl carbon Ij core level ionization, excellent quantitative agreement of the b parameters is found, both between the alternative calculations and between either calculation and experiment (see Section VLB.I). Given the spherical, therefore achiral, nature of the initial orbital in these calculations, any chirality exhibited in the angular distribution must stem from the final-state photoelectron scattering off the chiral molecular ion potential. Successful prediction of any non-zero chiral parameter is clearly then dependent on a reliable potential model describing the final state. At this level, there is nothing significant to choose between the potential models of the two methods. 

This chapter deals with the theory underlying the apphcation of wavepackets to molecular photodissociation and reactive scattering. The objective will be to derive and gather together the equations and theoretical methods needed in such calculations. No attempt will be made to reference aU calculations that have been undertaken in this very popular field. Several alternative related methods will be discussed, but it will not be possible to do full justice to all the different methods that have been proposed, many of which are being successfully used. 

An alternative to the addition method is the subtraction method. In this the entire system is treated using molecular mechanics and then quantum mechanics calculations and molecular mechanics calculations performed on the inner region or cluster. Usually there is no boundary region but link atoms are added to the inner region or cluster to make a molecule-like entity. The energy is then given by... 

As an alternative to the normal-mode method, Monte Carlo and molecular dynamics calculations have been performed on small clusters. Monte Carlo and molecular dynamics methods have the virtue of being exact, within calculable error bars, subject to the constraint of the approximate intermo-lecular interactions that are used. Prior to about six years ago both methods were restricted to systems projjerly described by classical mechanics. This restriction implied that systems for which tunneling or low-temjjerature vibrations were important at best could be treated approximately. 

The Htickel method predicts that in the case of a long polyene chain the longitudinal polarizability of n electrons varies as the cube of the molecular length, while the o polarizability should increase only linearly 18>. Refined molecular orbital calculations (taking into account bond alternation etc.) do not exhibit this abnormal asymptotic behaviour. 

An alternative is the sequential use of ab initio quantum chemistry and molecular simulation calculations. In this method quantum chemistry calculations are used to compute points on the potential energy surface of two interacting molecules, which are then fit to a potential form suitable... 

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