Coupled cluster calculations in the (0,2) and (2,0) sectors of the Fock space for the lowest electronic states of the O_2 molecule?

摘要

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 O_2. 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 X~3∑_g~- and two lowest excited states, a~1?_g and b~1∑_g~+, of the O_2 molecule. Due to a closed shell nature of the O_2~(-2) reference function, the PECs could be easily extended to the ca. 6 ? 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 O_2~(+2) ion (isoelectronic with the N_2 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.