Semiempirical Quantum Chemistry

The conformations of 3-substituted-2-benzothiazolinones have been studied by UV and IR spectroscopy and the quantum chemistry semiempirical method AMI <93Mi 306-06,93MI 306-07). 5-Acyl-2-(7V,7V-disubstituted amino)thiazoles unsubstituted or with a phenyl group at the 4-position, in solution exist predominantly as the carbonyl 0,S-syn rotamers (25a). On the other hand, a t-butyl group in the same position induces the 0,S-anti arrangement (25b) (Equation (3)) with the 4-methyl derivatives both forms are present <87JCS(P2)2305, 88JCS(P1)2209>. 

By combining classical samplings with quantum chemistry semiempirical TDHF calculations the impact of dynamic fluctuations on the first hyperpolarizability of helical strands has been evidenced . In particular, these fluctuations are responsible for relative variations of 20% in the hyper-Rayleigh responses in both pyridine-pyrimidine (py-pym) and hydrazone-pyrimidine (hy-pym) strands. Dynamical disorder has an even more important impact on the electric field-induced second harmonic generation responses, whose variations can reach 2 (py-pym) or 5 (hy-pym) times their mean value. These results demonstrate that geometrical fluctuations have to be taken into account for a reliable description of the second-order NLO properties in flexible structures such as helical strands. This work has also highlighted the relationships between the nature of the unit cell and the helical conformation of foldamers and their second-order NLO responses. In particular, the value of the hyper-Rayleigh depolarization ratio, which is characteristics of octupolar symmetry, is related to the helix periodicity, of three unit cells per turn in both compounds. 

The Slater-type orbitals were the first to be used in the molecular quantum chemistry semiempirical calculations. Unfortunately, such functions are not suitable for fast calculations of multicenter integrals in ab-initio calculations. Gaussian-type functions (GTFs) were introduced to remedy the difficulties. GTFs are used in basis sets in practically all modern codes for LCAO calculations of molecules. We know two exclusions... 

Stewart HP. MOPAC A semiempirical molecular orbital program. Quantum Chemistry Program Exchange, 1983. Prog. 455. 

When quantum calculations, at the ab initio and at the semiempirical level, gained foot in the realm of chemistry, a steadily increasing number of experimentalists began to use quantum calculations as a supplement in the exposition of their findings. In many case this was - and still is - nothing more that an ornament, like decorations on a cake. This use of quantum chemistry has been, in general, harmless, because results in contrast with experimental evidence have been rarely published, and this production may be considered now as a sort of advertising for the new-born computational chemistry. A more serious use of the facilities offered by the computational techniques is done by scientists provided of... 

A second situation where core electrons may be avoided is in the field of semiempirical quantum chemistry. In these approaches, the HF or KS operators... 

The usefulness of quantum-chemical methods varies considerably depending on what sort of force field parameter is to be calculated (for a detailed discussion, see [46]). There are relatively few molecular properties which quantum chemistry can provide in such a way that they can be used directly and profitably in the construction of a force field. Quantum chemistry does very well for molecular bond lengths and bond angles. Even semiempirical methods can do a good job for standard organic molecules. However, in many cases, these are known with sufficient accuracy a C-C single bond is 1.53 A except under exotic circumstances. Similarly, vibrational force constants can often be transferred from similar molecules and need not be recalculated. 

J. N. Murrell and A. J. Harget, Semi-empirical Self-consistent Molecular Orbital Theory of Molecules, Wiley-Interscience, London, 1972 G. H. Wagniere, Introduction to Elementary Molecular Orbital Theory and to Semiempirical Methods, Springer-Verlag, Berlin, 1976 J. Sadleij, Semi-empirical Methods of Quantum Chemistry, Wiley, New York, 1985. 

If quantum theory is to be used as a chemical tool, on the same kind of basis as, say, n.m.r. or mass spectrometry, one must be able to carry out calculations of high accuracy for quite complex molecules without excessive cost in computation time. Until recently such a goal would have seemed quite unattainable and numerous calculations of dubious value have been published on the basis that nothing better was possible. Our work has shown that this view is too pessimistic semiempirical SCF MO treatments, if properly applied, can already give results of sufficient accuracy to be of chemical value and the possibilities of further improvement seem unlimited. There can therefore be little doubt that we are on the threshold of an era where quantum chemistry will serve as a standard tool in studying the reactions and other properties of molecules, thus bringing nearer the fruition of Dirac s classic statement, that with the development of quantum theory chemistry has become an exercise in applied mathematics. 

The semiempirical molecular orbital (MO) methods of quantum chemistry [1-12] are widely used in computational studies of large molecules. A number of such methods are available for calculating thermochemical properties of ground state molecules in the gas phase, including MNDO [13], MNDOC [14], MNDO/d [15-18], AMI [19], PM3 [20], SAMI [21,22], OM1 [23], OM2 [24,25] MINDO/3 [26], SINDOl [27,28], and MSINDO [29-31]. MNDO, AMI, and PM3 are widely distributed in a number of software packages, and they are probably the most popular semiempirical methods for thermochemical calculations. We shall therefore concentrate on these methods, but shall also address other NDDO-based approaches with orthogonalization corrections [23-25],... 

Thiel, W. 2000. Semiempirical Methods in Modern Methods and Algorithms of Quantum Chemistry, Proceedings, 2nd Edn., Grotendorst, J., Ed., NIC Series, Vol. 3, John von Neumann Institute for Computing Jiilich, 261. 

The methods available for computing enthalpies of formation fall into two general groups those based on purely empirical schemes and those founded on quantum chemistry. The quantum chemical methods can be further divided into three types ab initio molecular orbital theory, density functional theory, and semiempirical molecular orbital theory. A summary of the types of method used to calculate enthalpies of formation is given in Table 2, along with some specific examples. This is not meant to be a comprehensive tabulation, but rather a list of some of the more popular approaches in use today. Table 3 names some of the commercially available computer programs having capabilities to calculate thermochemical data. 

Theoretical chemistry at UBC was further strengthened with the arrival of Delano Chong and Keith Mitchell in 1965 and 1966, respectively. Chong s interests in quantum chemistry have spanned the full range from semiempirical to ab initio molecular orbital methods. His long-standing interests in perturbation methods and constrained variations have figured prominently in his publications. He is probably best known for his attempts to calculate the X-ray and UV photoelectron spectra of molecules, often by means of perturbation corrections to Koopmans theorem.40 More recently he has shifted his focus to coupled pair functional methods and density functional methods, with a special interest in polarizabilities and hyperpolarizabilities.41... 

Chapter 1 outlined the tools that computational chemists have at their disposal, Chapter 2 set the stage for the application of these tools to the exploration of potential energy surfaces, and Chapter 3 introduced one of these tools, molecular mechanics. In this chapter you will be introduced to quantum mechanics, and to quantum chemistry, the application of quantum mechanics to chemistry. Molecular mechanics is based on classical physics, physics before modern physics one of the cornerstones of modem physics is quantum mechanics, and ab initio (Chapter 5), semiempirical (Chapter 6), and density functional (Chapter 7) methods belong to quantum chemistry. This chapter is designed to ease the way to an understanding of... 

The quantum mechanical methods described in this book are all molecular orbital (MO) methods, or oriented toward the molecular orbital approach ab initio and semiempirical methods use the MO method, and density functional methods are oriented toward the MO approach. There is another approach to applying the Schrodinger equation to chemistry, namely the valence bond method. Basically the MO method allows atomic orbitals to interact to create the molecular orbitals of a molecule, and does not focus on individual bonds as shown in conventional structural formulas. The VB method, on the other hand, takes the molecule, mathematically, as a sum (linear combination) of structures each of which corresponds to a structural formula with a certain pairing of electrons [16]. The MO method explains in a relatively simple way phenomena that can be understood only with difficulty using the VB method, like the triplet nature of dioxygen or the fact that benzene is aromatic but cyclobutadiene is not [17]. With the application of computers to quantum chemistry the MO method almost eclipsed the VB approach, but the latter has in recent years made a limited comeback [18],... 

Matsuzawa, N. and Dixon, D.A. (1992) Semiempirical calculations of hyperpolarizabilifies for extended n-systems Polyenes, polyynes, and polyphenyls. International Journal of Quantum Chemistry, 44, 497-515. 

The recent progress of computational quantum chemistry has made it possible to get realistic descriptions of vibrational frequencies for polyatomic molecules in solution. The first attempt in this direction was made by Rivail el al. [1] by exploiting a semiempirical QM molecular model coupled with a continuum description of the medium to compute vibrational frequency shifts for molecular solutes. An extension to ab initio QM methods, including the treatment of electron correlation effects and electrical and mechanical anharmonicities, was then proposed [2 1] in the framework of the Polarizable Continuum Model (PCM). 

The highly specific behavior of transition metal complexes has prompted numerous attempts to access this Holy Grail of the semi-empirical theory - the description of TMCs. From the point of view of the standard HFR-based semiempirical theory, the main obstacle is the number of integrals involving the d- AOs of the metal atoms to be taken into consideration. The attempts to cope with these problems have been documented from the early days of the development of semiempirical quantum chemistry. In the 1970s, Clack and coworkers [78-80] proposed to extend the CNDO and INDO parametrizations by Pople and Beveridge [39] to transition elements. Now this is an extensive sector of semiempirical methods, differing by expedients of parametrizations of the HFR approximation in the valence basis. These are, for example, in methods of ZINDO/1, SAMI, MNDO(d), PM3(tm), PM3 etc. [74,81-86], From the... 

Storer, J.W., Giesen, D.J., Cramer, C.J., and Truhlar, D.G., Class IV charge models a new semiempirical approach in quantum chemistry,./. Comput.-Aided Mol. Design, 9, 87-110, 1995. 

V. A. Gubanov, V. P. Zhukov, and A. O. Litinskii, Semiempirical Molecular Orbital Methods in Quantum Chemistry, Nauka, Moscow, 1976. 

Joanna Sadlej, Semiempirical Methods of CNDO, INDO, and NDDO Quantum Chemistry, Panstwowe Wydawnictwo Naukowe, Warsaw, 1977. 

---