Quantum-chemical computer programs

Certainly, the cluster calculations require some modification of quantum-chemical computational schemes. This is primarily connected with the main drawback of the cluster approximation, consisting of artificial scission of the chemical bonds between the cluster and the rest of the lattice. In most cases, however, such modifications do not involve the principal points, and therefore widely used quantum-chemical computer programs can serve as the basis for the cluster calculations. Some quantum-chemical methods used in... 

Many types of quantum-chemical computer programs are available from Quantum Chemistry Program Exchange, Chemistry Department. Indiana University, Bloomington, Indiana 47401. On the Internet at http //www.QCPE.Indiana.edu/... 

In Chapter 7 we developed a method for performing linear variational calculations. The method requires solving a determinantal equation for its roots, and then solving a set of simultaneous homogeneous equations for coefficients. This procedure is not the most efficient for programmed solution by computer. In this chapter we describe the matrix formulation for the linear variation procedure. Not only is this the basis for many quantum-chemical computer programs, but it also provides a convenient framework for formulating the various quantum-chemical methods we shall encounter in future chapters. 

The final total energy is effectively at the QCISD(T)/6-3114-G(3df,2p) level if the different additivity approximations work well. The validity of these additivity approximations was investigated by performing complete QCISD(T)/6-31l4-G(3df,2p) calculations on the set of 125 test reactions and in most cases the additivity approximations were found to be satisfactory. All of the calculations required for G2 theory are available in the quantum chemical computer program Gaussian 94. ... 

Concerning quantum chemical computations, we have used the MOLE-COLE program [18a], for HF and MP2 type computations. The Molecular Dynamics simulations with analytical force fields have been performed with the DINAMICA program [18b], The MOLECOLE-DFT program [18c] has been used for both the DFT energy minimization and for the DFT-Molecular Dynamics. 

The currently available quantum chemical computational methods and computer programs have not been utilized to their potential in elucidating the electronic origin of zeolite properties. As more and more physico-chemical methods are used successfully for the description and characterization of zeolites, (e.g. (42-45)), more questions will also arise where computational quantum chemistry may have a useful contribution towards the answer, e.g. in connection with combined approaches where zeolites and metal-metal bonded systems (e.g. (46,47)) are used in combination. The spectacular recent and projected future improvements in computer technology are bound to enlarge the scope of quantum chemical studies on zeolites. Detailed studies on optimum intercavity locations for a variety of molecules, and calculations on conformation analysis and reaction mechanism in zeolite cavities are among the promises what an extrapolation of current developments in computational quantum chemistry and computer technology holds out for zeolite chemistry. 

Recently, Diercksen and Hall (1) presented the OpenMol Program a proposal for an open, flexible and intelligent software system for performing quantum chemical computations. Central to their proposal was the observation that there is a close relationship between an abstract data type operation and a production rule in a rule-based expert system. The aim of this paper is to explore the establishment of a sound theoretical foundation for this relationship. 

Before 1980, force field and semiempircal methods (such as CNDO, MNDO, AMI, etc.) [1] were used exclusively to study sulfur-containing compounds due to the lack of computer resources and due to inefficient quantum-chemical programs. Unfortunately, these computational methods are rather hmit-ed in their reliability. The majority of the theoretical studies under this review utilized ab initio MO methods [2]. Not only ab initio MO theory is more reliable, but also it has the desirable feature of not relying on experimental parameters. As a consequence, ab initio MO methods are apphcable to any systems of interest, particularly for novel species and transition states. 

Here, n corresponds to the principal quantum number, the orbital exponent is termed and Ylm are the usual spherical harmonics that describe the angular part of the function. In fact as a rule of thumb one usually needs about three times as many GTO than STO functions to achieve a certain accuracy. Unfortunately, many-center integrals such as described in equations (7-16) and (7-18) are notoriously difficult to compute with STO basis sets since no analytical techniques are available and one has to resort to numerical methods. This explains why these functions, which were used in the early days of computational quantum chemistry, do not play any role in modem wave function based quantum chemical programs. Rather, in an attempt to have the cake and eat it too, one usually employs the so-called contracted GTO basis sets, in which several primitive Gaussian functions (typically between three and six and only seldom more than ten) as in equation (7-19) are combined in a fixed linear combination to give one contracted Gaussian function (CGF),... 

The applications of quantum chemical calculations to biological systems has been made possible by huge advances in computer facilities and the creation of better computer programs, capable of handling large systems. This book describes some of the quantum chemical methods used for such calculations, together with some widely used computer programs. 

Chapter 1 gives a short description of ab initio methods, Hartree-Fock and post-Hartree-Fock, focusing on the Gaussian computer programs. Chapter 2 describes semi-empirical calculations and their applications to biological systems. Chapter 3 addresses itself to electrostatic properties of molecules, as determined by quantum-chemical methods. The density functional method is discussed in chapter 4. Chapter 5 compares theoretically obtained parameters to experimental data. 

A specialized MOPAC computer software package and, in particular, its PM3 quantum-chemical program has been successfully applied in calculations. The results of calculations have shown that both oxygen atoms form bonds with two more active carbon atoms of CP molecular cluster (so-called bridge model of adsorption). The total energy of system after a chemical adsorption at such active atoms is minimal. 

While the manipulations involved in the practice of one electron MO theory are simple, it is clear that, unless someone is well familiar with the intricacies involved, mistakes can easily be made. We hope that in Part II we have provided sufficient warning of the pitfalls which await the careless and/or inexperienced worker who tries to apply MO methodology to a chemical problem. Needless to say, the proliferation of canned computer programs capable of performing quantum mechanical calculations of varied degrees of sophistication, makes forays into the theoretical arena irresistible to nonexperts. Whether this will turn out to be a panacea or a source of confusion for the experimentalists remains to be seen. 

Quantum-chemical cluster models, 34 131-202 computer programs, 34 134 methods, 34 135-138 for chemisorption, 34 135 the local approach, 34 132 molecular orbital methods, 34 135 for surface structures, 34 135 valence bond method, 34 135 Quantum chemistry, heat of chemisorption determination, 37 151-154 Quantum conversion, in chloroplasts, 14 1 Quantum mechanical simulations bond activation, 42 2, 84—107 Quasi-elastic neutron scattering benzene... 

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