Theoretical force-field model

Kryachko ES, Karpfen A (2006) Theoretical force-field model for blue-shifted hydrogen bonds with fluoromethanes. Chem Phys 329 313-328... 

One of the force fields with geometry-dependent charges was mentioned above. A model with such charges (Eq. [24]) was employed by Hill and Sauer ° within the ab initio parameterized MM force field model for protonated aluminosilicates. Deficiencies of the scheme originate from the absence of a solid theoretical background for the model it is just a first-order Taylor expansion of atomic charges with respect to distances between bonded atoms, and it neglects the influence of the environment. In addition, its parameterization is hampered by the use of bond rather than atomic parameters. 

Goodman, J. M., Kahn, S. D., Paterson, I. Theoretical studies of aldol stereoselectivity the development of a force field model for enol borinates and the investigation of chiral enolate -face selectivity. J. Org. Chem. 1990, 55, 3295-3303. 

The framework vibrations of AIPO4-5 and the isostructuxal SSZ-24 as well as the effect of protonation and cation exchange on framework vibrations upon re-ammoniation of H-Y and dehydration of Ni-Y were demonstrated by Jacobs et al. [373] via in-situ IR experiments and, at the same time, related to theoretical studies of the correspondingly relaxed structures, using a valence force field model. 

Takayama, T., M. Yuri, K. Itoh, T. Baba and J. S. Harris, Jr. 2000, Theoretical analysis of unstable two-phase region and microscopic structure in wurtzite and zinc-blende InGaN using modified valence force field model. J. Appl. Phys, 88(2) pp. 1104-1110. 

The cohesion of the solid depends on the interatomic forces. It is usually described by means of phenomenological or semi-phenomenological theoretical models. In the framework of the valence-force-field model, the chemical bonds and their interactions are replaced by springs called force constants. The latter are related to the phenomenological coefficients used in elastic theory of solids, namely the elastic constants Cij. The Cij can be measured by means of Brillouin spectroscopy or ultrasonic techniques. The study of elastic properties is of great interest if some cell strain is involved in a physical phenomena (see Sec. 3 Applications). 

Besides the classical techniques for structural determination of proteins, namely X-ray diffraction or nuclear magnetic resonance, molecular modelling has become a complementary approach, providing refined structural details [4—7]. This view on the atomic scale paves the way to a comprehensive smdy of the correlations between protein structure and function, but a realistic description relies strongly on the performance of the theoretical tools. Nowadays, a full size protein is treated by force fields models [7-10], and smaller motifs, such as an active site of an enzyme, by multiscale approaches involving both quantum chemistry methods for local description, and molecular mechanics for its environment [11]. However, none of these methods are ab initio force fields require a parameterisation based on experimental data of model systems DPT quantum methods need to be assessed by comparison against high level ab initio calculations on small systems. 

Our present views on the electronic structure of atoms are based on a variety of experimental results and theoretical models which are fully discussed in many elementary texts. In summary, an atom comprises a central, massive, positively charged nucleus surrounded by a more tenuous envelope of negative electrons. The nucleus is composed of neutrons ( n) and protons ([p, i.e. H ) of approximately equal mass tightly bound by the force field of mesons. The number of protons (2) is called the atomic number and this, together with the number of neutrons (A ), gives the atomic mass number of the nuclide (A = N + Z). An element consists of atoms all of which have the same number of protons (2) and this number determines the position of the element in the periodic table (H. G. J. Moseley, 191.3). Isotopes of an element all have the same value of 2 but differ in the number of neutrons in their nuclei. The charge on the electron (e ) is equal in size but opposite in sign to that of the proton and the ratio of their masses is 1/1836.1527. 

The performance is (as expected) very good. MMX provides relative (and absolute) stabilities with a MAD of only 1.2 kcal/mol, which is better than the estimates from the combined theoretical methods in Table 11.31. Considering that force field calculations require a factor of 10 less computer time for these systems than the ab initio methods combined in Table 11.31, this clearly shows that knowledge of the strengths and weakness of different theoretical tools is important in selecting a proper model for answering a given question. 

Thus, in contrast to preceding MM approaches explicit treatment of electronic polarisability is integral to a semi-empirical QM approach and promises excellent prospects for quantitative theoretical modelling of carbohydrates across a range of condensed phase environments. The results of the PM3CARB-1 model do however indicate in line with classical force field approaches [65, 73] that perhaps greater... 

Infrared spectra are straightforward to predict theoretically, demanding development of a force field (FF) to determine frequencies and dipole derivatives for intensities. These parameters were initially obtained using empirically fitted force constants and simple models for transition dipoles (Krimm and Bandekar, 1986 Torii and Tasumi, 1996). 

Theoretical models include those based on classical (Newtonian) mechanical methods—force field methods known as molecular mechanical methods. These include MM2, MM3, Amber, Sybyl, UFF, and others described in the following paragraphs. These methods are based on Hook s law describing the parabolic potential for the stretching of a chemical bond, van der Waal s interactions, electrostatics, and other forces described more fully below. The combination assembled into the force field is parameterized based on fitting to experimental data. One can treat 1500-2500 atom systems by molecular mechanical methods. Only this method is treated in detail in this text. Other theoretical models are based on quantum mechanical methods. These include ... 

In this section, we give a brief overview of theoretical methods used to perform tribological simulations. We restrict the discussion to methods that are based on an atomic-level description of the system. We begin by discussing generic models, such as the Prandtl-Tomlinson model. Below we explore the use of force fields in MD simulations. Then we discuss the use of quantum chemical methods in tribological simulations. Finally, we briefly discuss multiscale methods that incorporate multiple levels of theory into a single calculation. 

---