Reduce degrees of freedom

In general, these models are able to fit asymmetrical data sets but require the use of added parameters (thereby reducing degrees of freedom). Also, some of the parameters can be seriously correlated (see discussion in [2, 3, 8]). Most importantly, these are empirical models with no correspondence to biology. 

Our calculated relaxation and dissociation dynamics of planar OCS are best understood by first examining these phenomena In the colllnear world where OCS Is not allowed to bend (J ). The reduced degrees of freedom allow for a detailed study of statistical behavior by means of surfaces of section as well as the validation of other techniques that can be readily extended to more degrees of freedom. 

The SES and ESP approximations include the dynamics of solute degrees of freedom as fully as they would be treated in a gas-phase reaction, but these approximations do not address the full complexity of condensed-phase reactions because they do not allow the solvent to participate in the reaction coordinate. Methods that allow this are said to include nonequilibrium solvation. A variety of ways to include nonequilibrium solvation within the context of an implicit or reduced-degree-of-freedom bath are reviewed elsewhere [69]. Here we simply discuss one very general such NES method [76-78] based on collective solvent coordinates [71, 79]. In this method one replaces the solvent with one or more collective solvent coordinates, whose parameters are fit to bulk solvent properties or molecular dynamics simulations. Then one carries out calculations just as in the gas phase but with these extra one or more degrees of freedom. The advantage of this approach is its simplicity (although there are a few subtle technical details). 

Figure 9.8 The experimental rotational PED for NO(v = 0) from the NO2 NO(v = 0) -I-O reaction at an excess energy of 1949 cm. The lines through the experimental points are the PST and the prior distributions. The prior distribution assumes two rotational degrees of freedom. Thus a plot of IN(I)/ 2I + 1)] yields a straight line at low energies. The reduced degrees of freedom due to angular momentum conservation in the PST causes the deviation and the much better agreement with experiment. Taken with permission from Reisler and co-workers (Hunter et al. 1993).

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