Hartree Fock molecular system simulations

In this article, we present an ab initio approach, suitable for condensed phase simulations, that combines Hartree-Fock molecular orbital theory and modem valence bond theory which is termed as MOVB to describe the potential energy surface (PES) for reactive systems. We first provide a briefreview of the block-localized wave function (BLW) method that is used to define diabatic electronic states. Then, the MOVB model is presented in association with combined QM/MM simulations. The method is demonstrated by model proton transfer reactions in the gas phase and solution as well as a model Sn2 reaction in water. 

Molecules are sufficiently heavy that their motions can be described quite accurately by classical mechanics. In condensed phases (solution or solid state), there is a strong interaction between molecules, and a reasonable description can only be attained by having a large number of individual molecules moving under the influence of each other s repulsive and attractive forces. The forces in this case are complex and cannot be written in a simple form such as the Coulomb or gravitational interaction. No analytical solutions can be found in this case, even for a two-particle (molecular) system. Similarly, no approximate solution corresponding to a Hartree-Fock model can be constructed. The only method in this case is direct simulation of the full dynamical equation. 

In quantum mechanics, we shall discuss the Hartree-Fock (HF) method, the semiempirical method, and, most importantly, the density functional method, which is used most often for large condensed-matter systems and can be easily applied to clay minerals. Then we shall discuss various types of classical methods based on molecular mechanics or statistical mechanics, such as molecular mechanics, molecular dynamics, and Monte Carlo simulations. Among these methods, Monte Carlo and molecular dynamics have been used many times to study clay mineral and its hydrates. More detail on these methods will be discussed in the next section. 

The rigorous Hartree-Fock method without approximations is too expensive to treat large systems such as large organic molecules. Thus semiempirical quantum chemistry methods, which are based on approximated Hartree-Fock formalism by inclusion of some parameters from empirical data, have been introduced to study systems that do not necessarily require the exact quantum solutions to understand the physicochemical properties and are, therefore, very important in simulating large molecular systems. 

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