A priori complete active space self consistent field localized orbitals: an application on linear polyenes

Abstract

A recently proposed a priori localization technique is used to exploit the possibility to reduce the number of active orbitals in a Complete Active Space Self Consistent Field calculation. The work relies on the fact that the new approach allows a strict control on the nature of the active orbitals and therefore makes it possible to include in the active space only the relevant orbitals. The idea is tested on the calculation of the energy barrier for rigid rotation of linear polyenes. In order to obtain a relevant set of data, a number of possible rotations around double bonds have been considered in the ethylene, butadiene, hexatriene, octatetraene, decapentaene, dodecahexaene molecules. The possibility to reduce the dimension of the active space has been investigated, considering for each possible rotation different active spaces ranging from the minimal dimension of 2 electrons in 2 π orbitals to the π-complete space. The results show that the rigid isomerization in the polyene molecules can be described with a negligible loss in accuracy with active spaces no larger than ten orbitals and ten electrons. In the special case of the rotation around the terminal double bond, the space can be further reduced to six orbitals and six electrons with a large decrease of the computational cost. An interesting summation rule has been found and verified for the stabilization of the energy barriers as a function of the dimension of the conjugated lateral chains and of the dimension of the active space.

Acknowledgement

This research has been supported by local funding of the University of Ferrara (nano & nano project) and by a grant of the Italian MIUR.

Notes

†Permanent address: Department of Chemistry, University of Bergen, Allegaten 41, 5007 Bergen, Norway.

Additional information

Notes on contributors

Manuel Sparta†

†Permanent address: Department of Chemistry, University of Bergen, Allegaten 41, 5007 Bergen, Norway.