Project Details The Computation of Experimental Structure and Properties of Small Molecules by Ab Initio Calculation

Project No.:
150-2-95
Start Date:
01 January 1995
End Date:
31 December 2001

Objective

This is the first project in theoretical chemistry that Commission I.5 has proposed. It is therefore appropriate to establish the current state of ab initio calculations for the molecules for which calculations are expected to be the most successful, namely small molecules. The objective is to examine the agreement obtained between different ab initio calculations and the best experimental data, and to produce a summary that defines the successful methods and draws attention to the generalizations that can be made and those that are not valid. It is proposed to work out a list of two-, three-, and four-atomic molecules, and even larger systems, in which the most sophisticated computational results for structure and spectroscopy are given and compared with the best experimental results available. The list will be finite and the study will provide a basis of fact that should enhance the knowledge of experimentalists about the demonstrated accuracy of ab initio calculations and provide a basis for theoretical extension to larger molecules. The list will provide a partial answer to the important question that is regularly asked by experimentalists “To what extent are ab initio calculations reliable?”. The study will also lead naturally into questions of nomenclature, symbols and standards, which will have to be resolved to provide consistency within the report.

Progress

The scope of the project originally proposed has been carefully evaluated. The results in the current literature do not permit a systematic comparison of different computational methods for specific molecular properties. There is too little consistency in the level of theory used to make the comparison useful. Accordingly, the goal was changed while remaining within the scope of the project. The new goal is to systematically calculate the most important physical properties of diatomic and selected triatomic molecules by the best current density-functional theory. The goal is to determine the predictive power of this recently developed quantum chemical method, using the experimental data as a gauge, and to show the usefulness of calculated properties for those unstable diatomic systems for which experimental information is incomplete.

A 79 page report was reviewed by the Commission at the 1997 General Assembly in Geneva. The report contained the calculated results for almost 200 diatomic molecules compared with experimental data. Many changes and additions were made to incorporate the Commissioner’s comments and a presentation of the updated report was made to the Commission in Berlin. The final report is in preparation and will be submitted to the Commissioners for approval. The goal is for publication in 2001.

Project completed – A Technical Report entitled ‘Quantumchemical B3LYP/cc-pvqz computation of ground-state structures andproperties of small molecules with atoms of Z £18 (hydrogen to argon) is published in PureAppl. Chem.73(9), pp. 1521-1553, 2001.