Roy E. Kari

Uniform quality Gaussian basis sets

A direct optimization technique was applied to determine uniformly balanced, optimum (4s2p), (6s3p), (8s4p), and (10s5p) Gaussian basis sets for the first row atoms. These bases formally correspond to 2G, 3G, 4G, and 5G function representations per symmetry type per shell. The basis sets are throroughly balanced and all satisfy a rather rigorous quality criterion in terms of the local properties of the energy hypersurface over the space of the orbital exponents.

Quality of Gaussian basis sets: Direct optimization of orbital exponents by the method of conjugate gradients

Expressions are given for calculating the energy gradient vector in the exponent space of Gaussian basis sets and a technique to optimize orbital exponents using the method of conjugate gradients is described. The method is tested on the (9s5p) Gaussian basis space and optimum exponents are determined for the carbon atom. The analysis of the results shows that the calculated one‐electron properties converge more slowly to their optimum values than the total energy converges to its optimum value. In addition, basis sets approximating the optimum total energy very well can still be markedly improved for the prediction of one‐electron properties. For smaller basis sets, this improvement does not warrant the necessary expense.

Configuration Interaction Wavefunctions and Computed Inversion Barriers for NH3 and CH3

Ground state LCAO‐MO‐SCF wavefunctions, using a large contracted Gaussian basis set, have been constructed for NH3 and CH3−. These wavefunctions were determined for four points along the inversion coordinates of the respective molecules, and were then improved by the method of configuration interaction (CI). In these CI calculations, the computed energies of NH3 and CH3− were obtained to within 0.18 hartree of their respective nonrelativistic limits. The inversion barrier, as computed with the CI wavefunctions, was 6.9 kcal/mole for NH3 and 2.8 kcal/mole for CH3−.

AB Initio molecular orbital calculations on the si2h4molecule

Abstract Ab initio molecular orbital calculations using an extended Gaussian basis set have been performed on C 2 H 4 , CH 2 SiH 2 and Si 2 H 4 . The species CH 2 and SiH 2 have also been examined. Geometries were partially optimized and the energy difference between the planar singlet and orthogonal or twist triplet geometries of Si 2 H 4 was studied in order to provide a measure of the strength of the Si-Si bond in this molecule. Mulliken population analyses were carried out on CH 2 CH 2 and SiH 2 SiH 2 , to further study the nature of the Si-Si double bond in comparison with the C—C double bond.

The effects of optimization and scaling of AO exponents on molecular properties

The combined effects of direct optimization and various scaling techniques of Gaussian AO basis sets on the calculated molecular wavefunctions were analyzed for two examples, the hydrogen fluoride and ammonia molecules. An 8s4p Gaussian basis set for the fluorine atom which had been rigorously optimized for the total atomic energy by the conjugate gradient method was employed. For nitrogen, a similar atom optimized 9s5p basis set was employed. Three basis sets obtained in the course of the exponent optimization, the final optimized basis set, and basis sets derived from the optimum AO basis sets by various scalings of the exponents were employed in molecular calculations. The total energies and numerous one‐electron properties were analyzed. The convergence of molecular one‐electron properties to their limiting values is much slower than the energy convergence, and basis sets optimized for energy may be improved in terms of their property predictions by rigorous optimization. The simple scaling procedure ...

The interdependence and optimization of Gaussian function representations for the fluorine atom

Three different series of Gaussian basis sets of different dimensions were optimized for the fluorine atom. The basis sets were chosen from a series of function spaces of increasing dimension and each set was subsequently optimized by the method of conjugate gradients. The interdependence of optimum s and p type basis functions and the effects of cross representations were analyzed. The relation between the optima of different basis spaces and the predictability of optimum basis sets were investigated. The ratios of the orbital exponents showed systematic variations when the dimension of the basis space was extended.

A comparative molecular orbital study on the low lying singlet and triplet states of ethylene oxide

Two cross sections of the energy hypersurface for the ethylene oxide molecule, one involving a ring distortion, the other rotation about the C-C bond, have been studied by the semi-empirical and non-empirical SCF molecular orbital methods. The results indicate a ring distorted equilibrium geometry for the excited states of the molecule and implicate the lowest triplet state as an intermediate in the C2H4+ O(3P)→ ethylene oxide reaction.The results of the various types of calculations are compared.

Gaussian basis sets for nitrogen; Properties of NH3

Abstract Energies, geometries and one-electron properties of NH 3 were calculated with 47 different sp basis sets. These properties were then compared with those calculated with a large unconstrained and uncontracted sp basis set, which for our purposes was deemed to represent the sp limit. Such comparisons in an aggregate form were used to form conclusions about the quality and reliability of all 47 non sp limit basis sets.