Semiempirical quantum chemical methods

In summary, computational quantum mechanics has reached such a state that its use in chemical kinetics is possible. However, since these methods still are at various stages of development, their routine and direct use without carefully evaluating the reasonableness of predictions must be avoided. Since ab initio methods presently are far too expensive from the computational point of view, and still require the application of empirical corrections, semiempirical quantum chemical methods represent the most accessible option in chemical reaction engineering today. One productive approach is to use semiempirical methods to build systematically the necessary thermochemical and kinetic-parameter data bases for mechanism development. Following this, the mechanism would be subjected to sensitivity and reaction path analyses for the determination of the rank-order of importance of reactions. Important reactions and species can then be studied with greatest scrutiny using rigorous ab initio calculations, as well as by experiments. 

The electronic coupling of donor and acceptor sites, connected via a t-stack, can either be treated by carrying out a calculation on the complete system or by employing a divide-and-conquer (DC) strategy. With the Hartree-Fock (HF) method or a method based on density functional theory (DFT), full treatment of a d-a system is feasible for relatively small systems. Whereas such calculations can be performed for models consisting of up to about ten WCPs, they are essentially inaccessible even for dimers when one attempts to combine them with MD simulations. Semiempirical quantum chemical methods require considerably less effort than HF or DFT methods also, one can afford application to larger models. However, standard semiempirical methods, e.g., AMI or PM3, considerably underestimate the electronic couplings between r-stacked donor and acceptor sites and, therefore, a special parameterization has to be invoked (see below). 

Figure 2. Experimental and simulated fluorescence Stokes shift function 5(f) for coumarin 343 in water. The curve marked Aq is a classical molecular dynamics simulation result using a charge distribution difference, calculated by semiempirical quantum chemical methods, between ground and excited states. Also shown is a simulation for a neutral atomic solute with the Lennard-Jones parameters of the water oxygen atom (S°). (From Ref. 4.)...

Semiempirical quantum-chemical methods can be subdivided into two groups. The first one covers so-called simple MO LCAO methods, of which extended Hiickel treatment (EHT) (7) and its modification by Anderson (ASED) (5), additionally taking into account core repulsion, are most widely used in chemisorption computations. Anderson s improvement of EHT was aimed at obtaining more reliable values of the total energy and at gaining an opportunity to optimize the geometry of chemisorption structures. The method was mainly used in calculations of chemisorption and catalysis on metals. Its validity for oxide systems, with their rather highly ionic bonds, is formally less justified. 

M. Szafran, M. M. Karelson, A. R. Katritzky, J. Koput, and M. C. Zerner, /. Comput. Chem., 14, 371 (1993). Reconsideration of Solvent Effects Calculated by Semiempirical Quantum Chemical Methods. 

CaY zeolite has been found either to act as a reagent to reduce stilbenes to 1,2-diarylethanes or to act as a catalyst and isomerize (Z)-stilbenes to the more stable E-form, depending on the number of Bronsted acid sites present in the zeolite. It has also been suggested that Z-to-E one-way isomerization of (Z)-stilbene through proton addition-elimination and electron-transfer processes occurs in acidic zeolite cavities. The isomerization reaction of (E)- and (Z)-a-phenylcinnamic acid molecules has been studied " at the level of semiempirical quantum chemical methods. The calculations revealed that the (Z)-a-phenylcinnamic acid is slightly more stable than the -isomer. Kinetic studies have been made on the thermal Z-E isomerization of C(40)-carotenoids, and the rotational barriers for Z-E isomerization of different proline analogues have been investigated by dynamic H NMR spectroscopy. The effects of... 

Stacked NA base pairs AMBER 4.1 with the force field of Cornell et al [16] best reproduces the ab initio stabilization energies and geometries. The success of the Cornell et al force field is probably due to the derivation of atomic charges. It must be also mentioned that this force field provides a better description of interaction energies of NA base pairs than any semiempirical quantum chemical method or even nonempirical ab initio technique of a lower quality than that of the MP2 procedure (DFT or ab initio HF methods). 

Sixt, S., Altschuh, I and Brtiggemann, R. (1996). Estimation of pka for Qrganic Oxyacids Using Semiempirical Quantum Chemical Methods. In Software Development in Chemistry - Vol. 10 (Gasteiger, J., ed.), Fachgruppe Chemie-Information-Computer (CIC), Frankfurt am Main (Germany), pp. 147-153. 

Hobza, P. Kabelac, M. Sponer, J. Mejzlik, R Vondrasek, J., Performance of empirical potentials (AMBER, CFF95, CVFF, CHARMM, OPLS, POLTEV), semiempirical quantum chemical methods (AMI, MNDO/M, PM3, and ab initio Hartree-Fock method for interaction of DNA bases comparison with nonempirical beyond Hartree-Fock results, J. Comp. Chem. 1997, 18, 1136-1150... 

Now we need a formula to interpolate between these two cases. A very similar situation appears in semiempirical quantum chemical methods, where -y has a simple form, given by the Klopman-Ohno approximation ... 

Dielectric continuum models have also been developed to be used in conjunction with ab initio as well as semiempirical quantum chemical methods. For a comprehensive discussion on dielectric continuum models in general and on its use in connection with quantum chemistry calculations in particular, the reviews by Cramer and Truhlar are highly recommended [58],... 

There are two commonly employed theoretical methods for the study of molecules. These are based on quantum chemical or semiclassical models of molecular structure. Quantum chemical models are further divided into two categories ab initio and semiempirical. Here we will look primarily at semiempirical quantum chemical methods, and specifically those that are based on molecular orbital (MO) theory. 

We do not include in this review semiempirical quantum chemical methods that do not initially solve the Fock equations for a set of molecular orbitals. This is a subject unto itself. Such methods create orbitals by a fixed ansatz, such as localized orbitals formed from hybrid atomic orbitals, avoid matrix multiplication and diagonalization, and can be developed up to third... 

There have been a large number of quantum chemical studies on cluster models representing zeolitic lattice and the molecules adsorbed over them. Semiempirical quantum chemical methods such as CINDO [8], MNDO [2], AMI [4], PM3 [5] have been used to study the structural parameters [9,10] as well as interaction with probe molecules such as H2O, CO, NH3, Pyridine and other organic reactants [11-13]. 

Semiempirical quantum-chemical methods in computational chemistry... 

This review of semiempirical quantum-chemical methods outlines their development over the past 40 years. After a survey of the established methods such as MNDO, AMI, and PM3, recent methodological advances are described including the development of improved semiempirical models, new general-purpose and special-purpose parametriza-tions, and linear scaling approaches. Selected recent applications are presented covering examples from biochemistry, medicinal chemistry, and nanochemistry as well as direct reaction dynamics and electronically excited states. The concluding remarks address the current and future role of semiempirical methods in computational chemistry. 

Chapter 21 - Semiempirical quantum-chemical methods in computational chemistry. Pages 559-580, Walter Thiel... 

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