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Os termos não ligados são computacionalmente mais intensivos. Uma escolha popular é a de limitar as interações para energias emparelhadas (''pairwise energies''). O termo de van der Waals é usualmente calculado com um [[potencial de Lennard-Jones]] e o termo electrostático com a [[lei de Coulomb]], embora ambos possam ser armazenados em buffer ou dimensionados por um fator constante para dar conta da [[polarizabilidade]] eletrônica e produzir melhor concordância com as observações experimentais.
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==Parametrização==
==Parameterization==
InAlém additionda toforma thefuncional functionaldos formpotenciais, ofcampos thede potentials,força forcedefinem fieldsum defineconjunto ade setparâmetros ofpara parametersdiferentes fortipos differentde typesátomos, ofligações atomsquímicas, chemicalângulos bondsdiedros, dihedrale anglesassim andpor so ondiante. TheOs conjuntos parameterde setsparâmetros aresão usuallygeralmente empiricalempíricos. <!--A force field would include distinct parameters for an [[oxygen]] atom in a [[carbonyl]] [[functional group]] and in a [[hydroxyl]] group. The typical parameter set includes values for [[atomic mass]], [[van der Waals radius]], and [[partial charge]] for individual atoms, and equilibrium values of [[bond length]]s, bond angles, and [[dihedral angle]]s for pairs, triplets, and quadruplets of bonded atoms, and values corresponding to the effective [[spring constant]] for each potential. Most current force fields parameters use a "fixed-charge" model by which each atom is assigned a single value for the atomic [[electrostatic charge|charge]] that is not affected by the local [[electrostatic]] environment; proposed developments in next-generation force fields incorporate models for [[polarizability]], in which a particle's charge is influenced by electrostatic interactions with its neighbors. For example, polarizability can be approximated by the introduction of induced dipoles; it can also be represented by [[Drude particle]]s, massless, charge-carrying virtual sites attached by a springlike [[harmonic oscillator|harmonic potential]] to each polarizable atom. The introduction of polarizability into force fields in common use has been inhibited by the high computational expense associated with calculating the local electrostatic field.
Although many molecular simulations involve biological [[macromolecule]]s such as [[protein]]s, [[DNA]], and [[RNA]], the parameters for given atom types are generally derived from observations on small [[organic compound|organic]] molecules that are more tractable for experimental studies and quantum calculations. Different force field parameters can be derived from dissimilar types of experimental data, such as [[enthalpy]] of [[vaporization]] ([[OPLS]]), [[enthalpy]] of [[Sublimation (chemistry)|sublimation]], [[Molecular dipole moment|dipole moments]], or various spectroscopic parameters.
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