Molecular angle bending terms

The second solution is used in the MOMEC program and the ligand field molecular mechanics (LFMM) method as implemented in DOMMINO. In both cases, instead of using an L-M-L angle bend term, explicit ligand-ligand 1,3-nonbonded interactions are permitted which, in the spirit of VSEPR theory or points on a sphere (POS), facilitate the treatment of any coordination number. 

The most extensive use of multibody potentials for the simulation of materials by means of ionic pair potentials has been the addition of bond angle bending terms to two-body potentials used to describe silicate and framework structured materials. - s jn addition, harmonic planarity restraining terms have been employed in the simulation of polyatomic anions (in, e.g., inorganic carbonates). Here, as in the organic molecular mechanics methodology described latei structural distortions about an expected geometry are realized at an enei etic cost. 

Intensive use of cross-terms is important in force fields designed to predict vibrational spectra, whereas for the calculation of molecular structure only a limited set of cross-terms was found to be necessary. For the above-mentioned example, the coupling of bond-stretching (f and / and angle-bending (B) within a water molecule (see Figure 7-1.3, top left) can be calculated according to Eq. (30). 

Terms in the energy expression that describe a single aspect of the molecular shape, such as bond stretching, angle bending, ring inversion, or torsional motion, are called valence terms. All force fields have at least one valence term and most have three or more. 

Molecular mechanics calculations use an empirically devised set of equations for the potential energy of molecules. These include terms for vibrational bond stretching, bond angle bending, and other interactions between atoms in a molecule. All of these are summed up as follows ... 

The molecular mechanics energy of a molecule is described in terms of a sum of contributions arising from distortions from ideal bond distances ( stretch contributions ), bond angles ( bend contributions ) and torsion angles ( torsion contributions ), together with contributions due to non-bonded (van der Waals and Coulombic) interactions. It is commonly referred to as a strain energy , meaning that it reflects the strain inherent to a real molecule relative to some idealized form. 

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