Coupled cluster calculations in the (0,2) and (2,0) sectors of the Fock space for the lowest electronic states of the O₂ molecule

† In honour of Rodney J. Bartlett on the occassion of an anniversary in his private and scientific life.

Abstract

The Fock space (FS) multireference coupled cluster (CC) method in the (0,2) and (2,0) sectors has been applied to study the ground and excited states of the oxygen molecule O₂. The considered FS sectors – when used for the neutral molecule – yield double ionisation potential (DIP) or double electron affinity (DEA) values. Once they are applied to the doubly negative/doubly positive ions the results provide description of the neutral molecule. In the current case the FS(0,2)-CC variant was used to generate potential energy curves (PEC) for the ground X³Σ⁻_g and two lowest excited states, a¹Δ_g and b¹Σ⁺_g, of the O₂ molecule. Due to a closed shell nature of the reference function, the PECs could be easily extended to the ca. 6 Å which is not very common for the system as complex as the oxygen molecule. A similar attempt has been made to use the FS(2,0)-CC scheme by doing calculations for the ion (isoelectronic with the N₂ molecule). The energy values obtained for the equilibrium bond length could be considered as acceptable; however, the PECs are limited to the short range of interatomic distances.

Keywords:

• double ionisation potential
• double electron affinity
• Fock space coupled cluster method
• potential energy curves
• oxygen molecule

Acknowledgements

This work has been supported by the Ministry of Science and Higher Education, Poland, under Grant No. 2011/01/B/ST4/06503. In addition, one of the authors (MO) would like to thank for the support of the TWING project, financed by the European Social Fund.

Notes

† In honour of Rodney J. Bartlett on the occassion of an anniversary in his private and scientific life.