Computational methods empirical force fields

In spite of its limitations, molecular mechanics (MM) is a technique that is widely used for the computation of molecular structures and relative stabilities. The advantage of MM over quantum mechanical methods is mainly based on the computational simplicity of empirical force field calculations, leading to a comparatively small computational effort for MM calculations. Therefore, even large... 

Schrodinger equation. When the molecule is too large and difficult for quantum mechanical calculations, or the molecule interacts with many other molecules or an external field, we turn to the methods of molecular mechanics with empirical force fields. We compute and obtain numerical values of the partition functions, instead of precise formulas. The computation of thermodynamic properties proceeds by using a number of techniques, of which the most prominent are the molecular dynamics and the Monte Carlo methods. 

In order to overcome the limitations of currently available empirical force field param-eterizations, we performed Car-Parrinello (CP) Molecular Dynamic simulations [36]. In the framework of DFT, the Car-Parrinello method is well recognized as a powerful tool to investigate the dynamical behaviour of chemical systems. This method is based on an extended Lagrangian MD scheme, where the potential energy surface is evaluated at the DFT level and both the electronic and nuclear degrees of freedom are propagated as dynamical variables. Moreover, the implementation of such MD scheme with localized basis sets for expanding the electronic wavefunctions has provided the chance to perform effective and reliable simulations of liquid systems with more accurate hybrid density functionals and nonperiodic boundary conditions [37]. Here we present the results of the CPMD/QM/PCM approach for the three nitroxide derivatives sketched above details on computational parameters can be found in specific papers [13]. 

B. van de Graaf and J. M. A. Baas, /. Comput. Chem., 5 314 (1984). Empirical Force Field Calculations. 23. The Lagrange Multiplier Method for Manipulating Geometries. Implementation and Applications in Molecular Mechanics. 

Other important and effective approaches to computational chemistry are those called Empirical Force-Field methods (EFF methods), based on a mechanistic view of the molecule in terms of force constants of bonds, bending, torsion and other special interaction terms. The set of force constants constitutes a field of empirical parameters used for the calculation of molecular geometries and energies. 

Substituent steric descriptor calculated from standard enthalpy of formation obtained by computational chemistry using empirical force field methods. It is defined by the following relationship [Beckaus, 1978 Giese and Beckaus, 1978] ... 

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