Fixed partial charge model

Rotational and Dipole Strength Calculations for the CH-Stretching Vibrations of L-alanine Using the Localized Molecular Orbital, Nonlocalized Molecular Orbital, Atomic Polar Tensor, and Fixed Partial Charge Models ... 

The fixed partial charge (FPC) model (35) assumes that the electrons follow the nuclear motion perfectly and serve merely to screen the nuclear charge. The dipole moment derivative contribution from each nucleus is (5R /9Ga)o where... 

The electrostatic embedding of MM partial charges as a perturbational potential in the Hamilton operator is a frequently used method to account for Coulombic interactions between QM atoms and MM point charges. As the respective potential contributions influence the quantum mechanical calculation and by that the molecular orbitals and the associated electron density, this approach is rated superior compared to a Coulombic interaction model utilising fixed partial charges. 

The fixed partial charge (FPC) model was first introduced by Deutsche and Moscowitz (1968) for polymers. Later on it was modified by Schellman (1973) to be used to predict the VCD of small molecules. 

An expression which is structurally similar to the last term of equation 26 appears in the (conceptionally simple) fixed partial charge (FPC) model of vibrational CD In... 

Second, there are significant sources of errors in both the force field and continuum dielectric model used to approximate solvation effects. As an example, polarization effects are neglected in force fields based on fixed partial charges. A recent extension of the LIECE model has emphasized the importance of quantum mechanics to capture these effects, in particular for sets of inhibitors vith significant differences in the number of formal charges [114]. 

The additional computational effort, compared to fixed partial charges, is rather minor, i.e., about 15% for simulations of neat water [10,11] (the authors consider the TIP4P and SPC/E water models). Figure 3 shows an application of this technique to oligo(ethyleneoxide) in water [12]. This simulation contained a 7mer immersed in 1000 water molecules at ambient... 

Approximate models include the coupled oscillator (CO), the fixed partial charge (FPC),i7>i - o the charge flow (CF), the ring current, the... 

From the point of view of electrostatic interactions (that are obviously of crucial impxjrtance for the description of ionic liquids), there are two main choices fixed partial charges and polarizable models. 

More recently, force fields that account approximately for electronic polarization due to the strong electrostatic fields around nucleic acid molecules have become available (Babin et al. 2006). Additional terms often in the form of field-induced dipoles on atoms are used to approximately describe the deformation of the electron cloud due to an electrostatic field. It has been shown that such force field models can improve MD simulations of DNA resulting in overall better agreement with available experimental data than the standard force field form with fixed partial charges on atoms and no additional polarization term (Babin et al. 2006). 

Approximate models include the coupled oscillator (CO), the fixed partial charge (FPC), the chaige flow (CF), the ring current, the dynamic polarizability (DP), the bond dipole (BD), the localized molecular orbital (LMO) the nonlocalized molecular orbital (NMO), the atomic polar tensor (APT), and the locally distributed origins gauge... 

The vast majority of CSP has been limited to using intermolecular potentials that lack explicit inclusion of polarization,although its importance has become a topic of interest.Nonpolarizable force fields, based on fixed partial charges or fixed atomic multipoles, must implicitly account for the 20-40% of the lattice energy attributable induction. " On the contrary, polarizable models such as the AMOEBA force field for organic molecules... 

In the rigid model of Shim et al., the united atom representation for the CH2 and CH3 groups is used. Hence, the cation is represented by a rigid body and its geometry (bond lengths, angles) was taken from experimental x-ray diffraction data obtained from crystals of [C2-mim][Br] (Elaiwi et al., 1995). The electrostatic description of the cation consists of fixed partial charges centred on the atoms (Hanke et al., 2001). The interaction potential in the system is represented by a sum of pairwise additive interatomic LJ and Coulombic potentials ... 

Because of the expense in determining the dipoles, alternative models for the polarizability have been introduced. One, due to Sprik and Klein [65, 66], does not include dipole polarizabilities on the atoms. Instead it spreads the partial charge on an atom amongst a few sites that are very close to it. The position of the sites is fixed relative to the atom, but their charges are treated as dynamical variables that are allowed to change during an MD simulation, subject to the constraint that the total charge on the atom remains constant. 

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