Collect. Czech. Chem. Commun. 2005, 70, 951-978

Coupled-Cluster Study of Spectroscopic Constants of the Alkali Metal Diatomics: Ground and the Singlet Excited States of Na2, NaLi, NaK, and NaRb

Pavel Neográdya, Péter G. Szalayb, Wolfgang P. Kraemerc and Miroslav Urbana,*

a Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences, Comenius University, Mlynská dolina, Bratislava, SK-84215 Slovakia
b Department of Theoretical Chemistry, Loránd Eötvös University, Budapest, H-1518 Hungary
c Max-Planck Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, Garching, POB 1523, D-85741 Germany


A newly implemented two-determinant coupled-cluster method including single and double excitation operators (TD-CCSD) is applied to calculations of spectroscopic constants of alkali metal diatomics. The equilibrium bond length, harmonic vibrational frequency, anharmonicity, the dissociation energy and excitation energies are derived from the potential curves calculated for the ground state X1Σ+u and the two singlet mono-excited states, A1Σ+u and B1Πu of Na2 as well as for the ground X1Σ+ and the singlet excited A1Σ+ and B1Π states of the heteronuclear NaLi, NaK, and NaRb molecules. Spectroscopic constants and excitation energies agree reasonably well with experiment. Our results demonstrate that the relatively simple CCSD method for the excited states represented by two-reference determinants is a viable technique. The computer time needed for an excited singlet state is practically identical to the time which a standard single-determinant-based CCSD calculation takes.

Keywords: Multireference coupled-cluster calculations; Incomplete active space; Complete active space; Potential energy curves; Singlet excited states; Na2, NaLi, NaK, and NaRb; Ab initio calculations; TD-CCSD.

References: 73 live references.