Experimental reference data bond angles

In Figure 15.7, we have plotted the normal distributions of the errors in the calculated bond angles. The main difference from the plots for bond distances in Figure 15.4 is that, for bond angles, the CCSD(T) distributions are less sharply peaked and similar to those of the MP2 model, reflecting the poorer quality of the experimental reference data for bond angles than for bond distances. 

Fig. 7.23 Comparison of calculated and observed (x-ray ) mean N-C(a)-C bond angles for 37 proteins selected as described by Jiang et al. (1997,G). This reference is also the source of the values plotted, which are the region-average values, and . Regions of cb/ri-space and region numbering are explained in the lower graph. Experimental values (x-ray) were taken from the Brookhaven Protein Data Bank Chemistry Department, Building 555 Brookhaven National Laboratory, Box 5000, Upton N.Y. 11973-5000). The calculated values were obtained as described in the text.

To better reproduce experimental data for bond angles with large deviations from their reference values, higher order terms may be added to Eq. [6]. In addition to the quadratic term, MM2 (and the MM2-based programs) use a quadratic and a sixth-power term (Eq. [8]),... 

Fig. 9 (a) Zero pressure isobar in the 1,4-polybutadiene melt. The line shows MD results from the chemically realistic model. The symbols show average densities in the MpT MC simulation for the optimal choices of parameters for different versions of the LJ-type interaction. From Strauch et al. [117]. (b) Comparison between experimental data for polybutadiene melts in the temperature range from 299 to 461 K symbols) and calculations using PC-SAFT dashed curves) or TPTl-MSA solid curves) models. Parameters of the fits are quoted in the figure, where m is the effective degree of polymerization, which is also treated as a fit parameter and a and refer to a nonbonded LJ (12,6) potential). The bond length potential is the FENE -h LJ potential of Sect. 2.2, and no bond angle potential is used. Adapted from Binder et al. [120]... 

The ECEPP-05 force field (Arnautova et al. 2006 and references therein) is adjusted to both new experimental data and quantum mechanical results. Like previous ECEPP force fields ECEPP-05 utilizes the fixed bond lengths and bond angles but has some distinctive features as compared to both ECEPP/3 version and many other popular force fields. The van der Waals term of the energy is modeled by using the 6-exp potential function... 

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