John Kendrick

The GAMESS-UK electronic structure package: algorithms, developments and applications

A description of the ab initio quantum chemistry package GAMESS-UK is presented. The package offers a wide range of quantum mechanical wavefunctions, capable of treating systems ranging from closed-shell molecules through to the species involved in complex reaction mechanisms. The availability of a wide variety of correlation methods provides the necessary functionality to tackle a number of chemically important tasks, ranging from geometry optimization and transition-state location to the treatment of solvation effects and the prediction of excited state spectra. With the availability of relativistic ECPs and the development of ZORA, such calculations may be performed on the entire Periodic Table, including the lanthanides. Emphasis is given to the DFT module, which has been extensively developed in recent years, and a number of other, novel features of the program. The parallelization strategy used in the program is outlined, and detailed speedup results are given. Applications of the code in the areas of enzyme and zeolite catalysis and in spectroscopy are described.

Configuration interaction calculations of the Auger spectrum of CH4, HF, H2O and CO

Configuration interaction calculations of the doubly-ionized states of CH4, HF, H2O and CO are described and the results correlated with the Auger spectra of these molecules. The basis M.O.s for the configuration interaction calculations are obtained from restricted Hartree-Fock calculations on a low-lying state of the doubly ionized molecule. In general, these calculations allow an assignment of the quite complex Auger spectra of these molecules.

A configuration interaction calculation of the potential energy surface of Li3

Large-scale configuration interaction calculations of the potential energy surface of Li3 are described. The major prediction which emerges is that triangular configurations are more stable than linear ones, with the 2B2 and 2A1 states having different equilibrium geometries but essentially the same energies. It is found that the most stable linear configuration is symmetric, unlike the prediction of restricted Hartree-Fock calculations. A fit of our calculated surface to a semi-empirical LEPS function is presented.

Configuration interaction calculations of the satellite peaks associated with Cls ionization of carbon monoxide

Large scale configuration interaction calculations of the first five 2Σ+ carbon 1s core hole states of CO are described. Configuration selection, by means of perturbation theory, from ≳30 000 bonded functions yields final transition energies in good agreement with experiment. The inadequacy of a CI expansion including only singly excited configurations from the RHF reference configuration, is shown. A small CI expansion, using selected configurations, yields calculated intensities sufficiently accurate to allow an assignment of the experimental spectrum which is not possible from the calculated transition energies alone.

Calculated energetics of torsional motion in six diphenyl molecules: benzophenone, diphenyl ether, diphenyl sulphide, diphenyl sulphone, diphenylmethane and biphenyl

The calculation of the energetics of torsional motion in a number of aromatic, diphenyl molecules using the ab initio SCF method is described. The molecules chosen represent linkages, such as ether, ketone, sulphide, sulphone, methylene and biphenyl, which are found in many aromatic polymers. The results of the calculations should provide a useful ‘benchmark’ for attempts at using molecular mechanics methods to study the motion of the polymer chain. Where details are available, the calculated torsional and geometric parameters are in good agreement with experiment.

A computational method of performing MC SCF calculations using bonded functions

Abstract A computational method for carrying out MC SCF calculations using as basis configuration the bonded functions of Boys et al. is described. These functions are commonly used in large scale CI calculations, and the method described here utilizes many of the techniques used in such calculations, particularly the use of a list symbolic matrix elements. The method is shown to have good convergence properties.

Correlation effects and the 1s hole satellite spectrum of N2

Abstract Large scale CI calculations are used to predict the positions of the low lying satellite peaks associated with 1s ionization of N 2 . Multiply excited configurations are shown to be important in the interpretation of the satellite spectrum and a new assignment of the third and fourth observed satellites is proposed. A small scale CI calculation, including those configurations found to be important from the large scale calculation is used to interpret most of the peaks in the satellite spectrum, and also to compute their intensities. It is found that configuration interaction in the ground state of N 2 results in a large change in the predicted intensity of the second satellite peak.

A theoretical interpretation of the satellite peaks associated with core electron ionization of N2O

The satellite spectra associated with both oxygen and nitrogen 1s ionization of nitrous oxide are interpreted using configuration interaction calculations whose basis orbitals are the MOs of the corresponding core hole state, obtained by restricted Hartree—Fock calculations. The most intense peaks in the spectra are assigned to π → π transitions, and their intensities are correlated with the atomic character of the π MOs of N2O.

A theoretical study, including correlation, of the low-lying states of the ethynyl radical

Calculations of the low-lying electronic states of C2H, using gaussian and Slater basis sets are described. The ground state (X2Σ) and first two excited states (A2Π, B2Σ) are described in the RHF approximation. Large-scale CI calculations are used to improve the description of the X2Σ and A2Π states. These latter calculations give a ground-state geometry which excellently reproduces the observed microwave rotational transition energy, and shows the greater correlation energy associated with the 2Σ than with the 2Π state. A number of one-electron properties, including hyperfine coupling constants, are calculated from the CI wavefunctions.

Correlation effects in the nitrogen 1s photoelectron spectrum of nitric oxide

Abstract An interpretation of the anomalous intensity ratio of the two peaks arising from nitrogen 1s ionization of nitric oxide is presented. It is found that correlation effects in both the ground state of NO, and the ionic states ( 3 Π, 1 Π) of NO + contribute to deviations of this intensity ratio from the “frozen orbital” value of 3 to the experimental value of 3.43. MC SCF calculations are described which result in an excellent prediction of this intensity ratio. THe satellite peaks associated with core ionization from NO are briefly discussed.