Gaussian orbitals, computation

Slater orbitals have been demonstrated to be more accurate than a similar number of gaussian functions for molecular orbital computations, but they are not as mathematically convenient to use. This is why it is preferable to gaussian fimctions even if larger numbers of functions are required. 

Ab initio molecular orbital calculations (using the Gaussian 80 computer program) on the barrier to pseudorotation (for the furanose ring) of two model compounds, 2-deoxy-/ -D- /) cero-tetrofuranosylamine (781) and 2-deoxy-2-fluoro-)3-D-erythrofuranosylamine (782) were reported. Al-... 

While in principle all of the methods discussed here are Hartree-Fock, that name is commonly reserved for specific techniques that are based on quantum-chemical approaches and involve a finite cluster of atoms. Typically one uses a standard technique such as GAUSSIAN-82 (Binkley et al., 1982). In its simplest form GAUSSIAN-82 utilizes single Slater determinants. A basis set of LCAO-MOs is used, which for computational purposes is expanded in Gaussian orbitals about each atom. Exchange and Coulomb integrals are treated exactly. In practice the quality of the atomic basis sets may be varied, in some cases even including d-type orbitals. Core states are included explicitly in these calculations. 

Interaction in Polyatomic Molecules Ab Initio Computations with Gaussian Orbitals. 

Compared with the STOs, there are both advantages and disadvantages to Gaussian orbitals. The main disadvantage is that larger basis sets are required for Gaussians on the other hand several of the integrals which arise in molecular calculations are easier to compute. 

The HF CO method is especially efficient if the Bloch orbitals are calculated in the form of a linear combination of atomic orbitals (LCAO)1 2 since in this case the large amount of experience collected in the field of molecular quantum mechanics can be used in crystal HF studies. The atomic basis orbitals applied for the above mentioned expansion are usually optimized in atoms and molecules. They can be Slater-type exponential functions if the integrals are evaluated in momentum space3 or Gaussian orbitals if one prefers to work in configuration space. The specific computational problems arising from the infinite periodic crystal potential will be discussed later. 

The FSGO (floating-spherical-Gaussian orbital) method in an ab initio computation procedure results in direct bond orbitals without base functions restricted to the atoms. When the degree of bond bending was calculated with this method, the following results were obtained ... 

The Lennard-Jones potential parameters a, p, Gap are chosen [20] to agree with diffraction experiments [17]. The charge q has been computed with the help of a linear combination of plane orbitals (LCPO) [37] or of Gaussian orbitals (LCGO). In both cases, q is significantly less than le qp = 0.1c in TeFe (LCPO) qp = 0.25e in TeFe (LCGO) [38]. 

The Institute first considered the fundamentals of molecular orbital computations and ab initio methods and the construction of Potential Energy Surfaces. These subjects were further explored in several applications related with optimization of equilibrium geometries and transition structures. Practical examples were studied in Tutorial sessions and solved in the computational projects making use of the Gaussian 88 and Gaussian 90 programs. 

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