Energy force field

The following sections give an overview of the functional form of the PFF and a short explanation of the various contributions to the total force field energy of a molecule or molecular system. 

The force constants in the equations are adjusted empirically to repro duce experimental observations. The net result is a model which relates the "mechanical" forces within a stmcture to its properties. Force fields are made up of sets of equations each of which represents an element of the decomposition of the total energy of a system (not a quantum mechanical energy, but a classical mechanical one). The sum of the components is called the force field energy, or steric energy, which also routinely includes the electrostatic energy components. Typically, the steric energy is expressed as... 

The force field energy is written as a sum of terms, each describing the energy required for distorting a molecule in a specific fashion. 

Jorgensen WL, Maxwell DS, Tirado-Rives J (1996) J Am Chem Soc 118 11225 Jorgensen WL, McDonald NA (1998) Theochem - J Molec Struct 424 145 CAChe molecular moldelling program (1996) Oxford Molecular Group Inc In molecular mechanics the force field energy E is usually termed the steric energy of the molecule... 

Forced-vibration instruments, 21 745 Force field calculations, 16 742 Force field energy, 16 742 Force field performance, 16 745 Force fields, 16 743-745 Force field simulations, 16 746-747 programs for, 16 746 Force modulation microscopy, 3 332 Forces, exponents of dimensions in absolute, gravitational, and engineering systems, 8 584t Forchlorfenuron, 13 43t, 53 Ford nuclear reactor, 17 594... 

The investigation of factors affecting facial selectivity in the hydroboration of steroidal -alkenes revealed the facial (a vs /3) stereoselectivities of hydroboration of androst-5-enes (69) and B-norandrost-5-enes (70) do not parallel the difference between the calculated force-field energies for a- and jS-cyclobutane models (71)-(74). This finding appears to suggest that the facial selectivity is not determined by the four-centre transition state but by the relative ease of formation of the initial tt-complex. ... 

The conceptually similar COMBINE approach was developed by Wade et al. [85] on the basis of the analysis of force-field energy contributions per amino acid to describe interaction differences to a congeneric set of ligands. The resulting predictive regression equations were also reported for applications in structure-based design projects [86-88]. 

The most common response nowadays is to supplement the experimental data with the highest quality ab initio data that can be had (either from molecular orbital or density functional calculations). A pleasant feature of using theoretical data is that one can compare regions on a PES that are far from equilibrium structures by direct computation rather than by trying to interpret vibrational spectra. Furthermore, one can attempt to make force-field energy derivatives correspond to those computed ab initio. The only limitation to this approach is the computational resources that are required to ensure that the ab initio data are sufficiently accurate. 

---