Semiempirical Crystal-Orbital Methods

It can be seen from Table 2.6 that the physically most important valence and conduction bands in the later ab initio calculations are much broader (0.435-0.789 and 0.245-0.820 eV, respectively) than those obtained by application of different semiempirical crystal-orbital methods. With the simple PPP-CO approximation, the corresponding values for the highest filled bands are 0.218-0.299 eV and those for the conduction band are about 0.109 eV. Energy-band-structure calculations for the base stacks taking into account the effect of the other valence electrons with the aid of the CNDO/2 CO method give again broader bands (valence bandwidths of 0.136-0.490 eV and conduction band-widths of 0.109-0.245 eV), while the MINDO/2 CO results indicate somewhat less-broad bands (valence bandwidths of 0.027-0.299 eV and conduction bandwidths of 0.027-0.163 eV). For futher details on the semiempirical crystal-orbital calculations see also Chapter 3. 

The preceding methods were applied to a number of periodic polymers with a small unit cell. Many of these calculations used different all-valence electron semiempirical crystal-orbital methods (CNDO/2, — INDO, — MINDO, and MNDO CO methods). At this point we shall discuss only some of the not very numerous ab initio vibrational calculations and also an MNDO CO work on fra/is-polyacetylene (PA), because in this investigation disorder in PA due to soliton and polaron formation was also taken into account. ... 

In this paper, we make use of molecular modelling techniques, particularly the AMI semiempirical molecular orbital method, to study the intermolecular interactions that are important for determining the manner in which crystal formation takes place. We are particularly inter ested in compounds that can potentially exhibit nonlinear optical properties. The calculational techniques are directed towards providing insight into the manner in which the desired nonlinear optical properties can be op timized in the macromolecular crystal state.(1)... 

Semiempirical valence electron molecular orbital methods were used to evaluate the electronic structure of derivatized phosphonitrilic compounds. All molecular structures were fully force optimized with no geometric constraints other than obvious symmetries. Structural parameters for initial geometries were taken from available x-ray crystal structure determinations whenever possible. Although pre-... 

The formation of C-C chemical bonds in a variety of solids, including some refractory dicarbides, has been considered by Li and Hoffman (1989) and Wijeyesekera and Hoffman (1984) based on EHT (extended Huckel theory) calculations. To our knowledge, these works are the only ones where the band analogues of bond populations, the so-called crystal orbital overlap populations (COOPs) have been calculated for refractory compounds. The most noticeable result is that, in spite of the evident crudeness of the nonself-consistent semiempirical EHT method, the calculations allow us to understand the nature of the phase transition from cubic to hexagonal structure which occurs in the ZrC, NbC, MoC,... series as the VEC increases. The increase of metal-to-metal bonding when going from cubic (NaCl-type) to hexagonal (WC-type) becomes evident. 

AMI Cambridge Structural Database Carbohydrate Force Fields Conformational Analysis 1 Conformational Analysis 2 Conformational Analysis 3 Continuum Solvation Crystal Structure Calculations 1 Crystal Structure Calculations 2 Cyclodextrins Force Fields MM3 Hydrogen Bonding 1 Hydrogen Bonding 2 Hydrogen Bonds Semiempirical Methods Hyperconjugation MNDO Natural Bond Orbital Methods PM3 Solvation Carbohydrates. 

Due to the central role of DNA and proteins in biochemistry and biophysics the computation of the electronic structure of periodic polymers built from nucleotide bases, base pairs, nucleotides and amino acids, respectively, had been of high interest since about twenty years. Early calculations of the band structure of DNA related periodic polymers have been performed with the crystal orbital (CO) method on the basis of different semiempirical levels (1). Recently the results of ab initio Hartree-Fock CO (2, 3) band structure calculations for the four nucleotide base stacks (4-6), the two Watson-Crick base pair stacks (6), the sugar-phosphate chain (4,5) and the three nucleotides cytidine (4,5), adenylic acid and th3nnidine (6) have been reported. These computations represent a significant progress but the following improvements are required for a more accurate description of the electronic structure of real DNA and its transport properties ... 

Any orbital-based scheme can be used for crystal-structure calculations. The trend is toward more accurate methods. Some APW and Green s function methods use empirical parameters, thus edging them toward a semiempirical classification. In order of preference, the commonly used methods are ... 

Seven-membered cyclic polysulfides are as a rule in chair conformation both in crystal (see Section 9.17,3.1) and in solution, because these conformers have the lowest torsion strain. Experimental ring inversion energy barriers for 6-carbomethoxymethyl-l,2,3,4,5-benzopentathiepin ( ) and unsymmetric pyrazolo-2//-l,3,4,5- or pyrazolo-4//-l,2,3,5-tetrathiepins (45) and (47) are 19.3 and 18.5 kcal mol, respectively <87JOC24ii>. Molecular orbital calculations, which have been carried out for 1,2,3,4,5-pentathiepin (62), 1,2,3,4,5-benzopentathiepin (35), and thieno[3,4-/]-1,2,3,4,5-pentathiepin (37) by means of the semiempirical AMI method <88JST(167)245> and for the pen-... 

The aim of this chapter is to provide an overview of the theories and methodologies governing MM of crystal structure and surfaces, growth morphology, and the effects of impurities and additives on crystallization. Different computational methods are used in these modelings, including ab initio or molecular orbital calculations, semiempirical methods, molecular mechanics, molecular dynamics, and Monte Carlo (MC) simulation. The reader is recommended to some excellent reference textbooks on the principles of MM and simulation (Hinchliffe 2003 Leach 2001 Myerson 1999). 

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