Molecular Models (Force Fields)
Molecular simulation studies require the accurate calculation of the potential energy of the system as function of its configuration and the structures of the molecules.
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Molecular Models (Force Fields)
Molecular simulation studies require the accurate calculation of the potential energy of the system as function of its configuration and the structures of the molecules. Theoretically, ab initio quantum mechanical calculations can be employed to determine these properties. However, ab initio simulations are still restricted to systems of some 100 atoms and cannot be used to calculate properties in the condensed phase. Thus, molecular simulations are primarily performed using simplified analytical potential energy functions, so called force fields, that relate the chemical structure of the system to its conformational energy. In this chapter, we will mainly focus on classical force fields for fluid phases and their parametrization. The classical molecular models (first and second generation) use fixed partial charges to describe the charge distribution within the molecules. However, in Sect. 6.2 we will also discuss approaches to introduce polarization into the force field (third generation). Section 6.3 provides a short overview on other force field types such as potentials for metals or reactive force fields. In Sect. 6.4, we will finally discuss specific aspects on molecular modeling, i.e. approaches to account for many-body effects and the influence of intermolecular flexibility on the description of thermophysical properties of water in Sect. 6.4. The discussion on force fields in this chapter concentrates on so called all-atoms models that consider all atoms including hydrogen. Though most conclusion also apply for united-atom models in which hydrogens bonded to carbons are combined with them to single interaction sides. However, coarse graining models in which larger groups of atoms are presented by pseudo atoms are not within the scope of this discussion.
6.1 Classical Force Fields for Condensed Phases For most force fields used in condensed phase simulations, the configurational energy Uconf is described by the following standard functional form [77] © Springer Nature Singapore Pte Ltd. 2017 G. Raabe, Molecular Simulation Studies on Thermophysical Properties, Molecular Modeling and Simulation, DOI 10.1007/978-981-10-3545-6_6
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6 Molecular Models (Force Fields)
Uconf =
kr (r − r0 )2 +
bonds
+
kθ (θ − θ0 )2
(6.1)
angles
kφ [1 + cos(nφ − δ)]
dihedrals
+
kω (ω − ω0 )2
improper
+
electrostatics
qi qj 4πε0 rij
+
vdW
εij
Rmin,ij rij
12
Rmin,ij −2 rij
6 .
It accounts for additive interactions between the atoms (or sites) of the system’s molecules, with the atoms treated as point masses centered on their nuclei. Molecular models following this simple functional form are referred to as class I force fields. This class includes the AMBER [22], OPLS [60–62], CHARMM [35, 78] or the GROMOS [113] force fields that are widely used in studies on biological and organic molecules such as proteins or nucleic acids. Beyond that, many force fields to compute thermophysical properties in chemical engineering, amongst others for compo
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