Dieter Poppinger

On the calculation of transition states

Abstract An algorithm has been developed to calculate transition state geometries directly in the framework of ab initio single-determinant molecular orbital theory. As an example, the procedure is applied to the rearrangement of ethenylidene to acetylene.

Geometry optimization in ab initio molecular orbital theory

Abstract A gradient optimization procedure has been used to calculate equilibrium geometries for several small molecules in the framework of ab initio molecular orbital theory. The gradient method was found to be faster and more reliable than two direct search procedures.

A comparative study of the equations of motion, and configuration interaction methods for the calculation of excitation properties of methane and water

In this paper we provide a direct comparison (over the same molecular basis) of the Equations of Motion (EOM) method and Configuration Internation (CI) methods, for the calculation of excitation properties of water and methane. We find that when double particle-hole interactions have been included in the EOM method and double excitations in the CI method, the results from the EOM method are in closer agreement with the experimental values than the results from the CI method. Furthermore, we find the EOM method to be more economical and convenient, from a computational point of view, than the corresponding CI method. The results from the EOM method also tend to be less sensitive to the choice of basis set than the results from the usual CI treatment.

The nature of the C…C ring-opened form of the ethylene oxide radical cation

CAS SCF and Moller-Plesset perturbation calculations with polarized basis sets have been used to investigate the nature of the C…C ring-opened isomer of the ethylene oxide radical cation. The best calculations predict a planar C 2V structure for CH 2 OCH 2 + , lying 82 kJ mol −1 below the ring-closed ethylene oxide radical cation. However, the possibility of a slightly distorted structure, lying in a very shallow potential well, cannot be completely ruled out.

On the Jahn—Teller distortion in the cyclopropenyl radical

Abstract Ab initio LCAO SCF MO and Cl calculations have been carried out on the cyclopropenyl radical, C3H3. In accordance with the Jahn—Teller th

Structural and energetic predictions for anions from a modified Hartree—Fock procedure

Abstract The performance of a modifed Hartree—Fock (MHF) procedure which differs from conventional HF theory through the inclusion of continuum functions in t

The structure of the cyclobutane radical cation

Abstract Ab initio molecular orbital theory has been used to study the structure of the cyclobutane radical cation (CB +. ). In accordance with the Jahn-Teller theorem, distortions from D 4h symmetry (1) are found. The preferred structures correspond to first-order Jahn-Teller distortions, leading to rectangular (2) and rhomboidal (3) geometries, the latter being marginally favored. Second-order Jahn-Teller distortions lead to trapezium (4) and kite (5) forms which are of somewhat higher energy than 2 and 3. The potential surface linking 2–5 is quite flat indicating considerable structural flexibility for CB +. .

Geometry optimizations with a small contracted gaussian basis set

Abstract The STO-2G basis set has been used to calculate equilibrium geometries of 27 molecules. A comparison with STO-3G and experimental results indicates that the exploration of potential energy surfaces of large organic molecules can be efficiently carried out using the STO-2G basis set.