Campo de força (química): diferenças entre revisões
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Os conjuntos de parâmetros e as formas funcionais são definidos pelos desenvolvedores dos potenciais interatômicos como sendo [[Consistência|auto-consistente]]s. Uma vez que as formas funcionais dos termos potenciais variam extensivamente entre os potenciais interatômicos mesmo os estreitamente relacionados (ou versões sucessivas de um mesmo potencial interatômico), os parâmetros de uma função potencial interatômica devem claramente nunca ser usados em conjunto com uma outra função potencial interatômica.
==Deficiências==
▲All [[interatomic potentials]] are based on numerous approximations and derived from different types of experimental data. Therefore they are called ''empirical''. Some existing energy functions do not account for electronic [[Dielectric polarization|polarization]] of the environment, an effect that can significantly reduce electrostatic interactions of partial atomic charges. This problem was addressed by developing "polarizable force fields" <ref name="Ponder">Ponder JW and Case DA. (2003). Interatomic potentials and their relative parameters for protein simulations. ''Adv. Prot. Chem.'' '''66''' 27-85.</ref><ref name="warshel">Warshel A, Sharma PK, Kato M and Parson WW (2006). "Modeling Electrostatic Effects in Proteins." ''Biochim. Biophys. Acta'' '''1764''' 1647-1676.</ref> or using macroscopic [[dielectric constant]]. However, application of a single value of [[dielectric constant]] is questionable in the highly heterogeneous environments of proteins or biological membranes, and the nature of the dielectric depends on the model used.<ref name="Shultz">Schutz CN. and Warshel A. (2001). "What are the dielectric "constants" of proteins and how to validate electrostatic models?". ''Proteins'' '''44''' 400-417.</ref>
All types of [[Van der Waals force]]s are also strongly environment-dependent, because these forces originate from interactions of induced and "instantaneous" dipoles (see [[Intermolecular force]]). The original [[Fritz London]] theory of these forces can only be applied in vacuum. A more general theory of [[Van der Waals force]]s in condensed media was developed by A. D. McLachlan in 1963 (this theory includes the original London's approach as a special case).<ref name="Israelachvili">Israelachvili, J.N. (1992). ''Intermolecular and surface forces.'' Academic Press, San Diego.</ref> The McLachlan theory predicts that van der Waals attractions in media are weaker than in vacuum and follow the "like dissolves like" rule, which means that different types of atoms interact more weakly than identical types of atoms.<ref name="Leckband">Leckband, D. and Israelachvili, J. (2001). "Intermolecular forces in biology". ''Quart. Rev. Biophys.'' '''34''' 105-267.</ref> This is in contrast to "combinatorial rules" or Slater-Kirkwood equation applied for development of the classical force fields. The "combinatorial rules" state that interaction energy of two dissimilar atoms (e.g. C…N) is an average of the interaction energies of corresponding identical atom pairs (i.e. C…C and N…N). According to McLachlan theory, the interactions of particles in a media can even be completely repulsive, as observed for liquid [[helium]].<ref name="Israelachvili" /> The conclusions of McLachlan theory are supported by direct measurements of attraction forces between different materials ([[Hamaker constant]]), as explained by [[Jacob Israelachvili]] in his book "Intermolecular and surface forces". It was concluded that "''the interaction between hydrocarbons across water is about 10% of that across vacuum''".<ref name="Israelachvili" /> Such effects are unaccounted in the standard molecular mechanics.
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