Fock space coupled cluster study of the 1¹Π_<i>g</i> state of the Li₂ molecule

<p>Recently reported (M. Musiał, <i>J. Chem. Phys.</i>, <b>136</b>, 134111 (2012)) multireference coupled cluster method formulated in the (2,0) sector of the Fock space proved to be an efficient scheme in the treatment of potential energy curves (PECs) for alkali metal diatomics Me₂. The (2,0) sector provides description of states obtained by an attachment of two electrons to the reference which for the Me₂ is a doubly ionised Me^{+ 2} ₂ system. The latter has a very concrete advantage in the calculations of the PECs since it dissociates into closed shell fragments (Me^{+ 2} ₂ → Me⁺+Me⁺); hence, the restricted Hartree–Fock reference can be used in the whole range of interatomic distances. In this work, an accurate PEC and vibrational energy levels are obtained for the state of the lithium dimer. The average deviation of the vibrational term values from experiment is 0.79 wavenumber and is significantly lower than that provided by other theoretical works.</p

Excited and ionized states of the ozone molecule with full triples coupled cluster methods

<div>The role of connected triple excitations in coupled cluster CC calculations of vertical excitation energies, ionization potentials, and the electron affinity of the ozone molecule is evaluated. The equation of motion EOM and Fock space FS multireference CC approaches with full triples have</div><div>been used in the calculations. The effect of the T3 and R3 operators significantly improve the EOM CCSD results for all considered quantities. A similar behavior is observed in the case of the FS-CC calculations. The FS-CC calculations with full triples have been obtained only for the intermediate</div><div>Hamiltonian realization of the FS approach as the standard formulation diverges. The latter results are rigorously linked, and less expensive since smaller matrices are diagonalized.</div