Ab-initio Description of Triplet States

Before discussing the elaborate ab-initio methods now at our disposal, let us recall that less sophisticated tools are also available. These are not dealt with here, both because they have already been the subject of extensive reviews and because they have not yet reached the state of refinement that would permit their systematic and reliable use. Some of the results will, however, be mentioned in the second part of this article. These tools may be classified in three groups. 

Completely empirical treatments, in which the potential energy surfaces of singlet and triplet states are represented as a function of potential parameters fitted to the available experimental information (equilibrium geometries, vibrational frequencies), have had a considerable success for molecules for which a localized electronic description is applicable -io). In the case of conjugated molecules, the important delocalization of the tc electrons introduces difficulties in such treatments and the two following approaches appear preferable  

The first approach uses semi-empirical procedures (EHT, PPP, CNDO, INDO, MINDO) 12-16) Many calculations of this type have been performed and remarkable results have been obtained in the evaluation of ground-state properties 12). These methods have also been tested in the study of triplet-state potential energy surfaces (tetramethylene retinal i8-i9) acrolein 20), benzophenone barrelene 22) prop5mal and formic acid 23) acetone 24) formaldehyde 25)). The MINDO/2 method seems particularly promising 26-27). 

The ab-initio SCF methods employed in the study of excited states may be classified in three groups depending on the nature of the molecular orbitals (MO s) used to build the exdted-state wave function. 

Where the available theoretical results allow, all the methods to be discussed are illustrated by the same example the mr triplet state of the formaldehyde molecule. In its ground state formaldehyde is a planar molecule of electronic configuration o)  

---