MRCI Investigation On Spectroscopic Properties For Some Small Diatomic Molecules

Molecular potential energy curve (PEC) and molecular spectroscopy are one of the important branches in atomic and molecular physics. It is the foundation to investigate atomic and molecular collision and reaction, and plays a special role in the atom-cluster growth, dissociation and stability analyses.In the present work, the spectroscopic parameters and molecular constants for 4 diatomic molecules (CN, BO, GeO and GeS) are investigated in detail.First, the PECs of the CN (X2Σ+ andΛ2Π), BO (X2Σ+, A2II, B2Σ+ and C2Π) and GeO (X1Σ+, A1Π, C1Σ- and b3Π) are calculated employing the complete active space self-consistent field (CASSCF) method followed by the valence internally constracted multireference configuration interaction (MRCI) approach in combination with the Dunning's correlation consistent basis sets in MOLPRO 2008.1 program package. With the aid of module VIBROT in Molcas program package, the PECs are fitted and the spectroscopic parameters (De, Re,ωe,ωexe, ae and Be) are determined for each electronic state. The complete vibrational states are also obtained. For each vibrational state, the vibrational level G(t>), inertial rotation constant B0 and centrifugal distortion constant D0 are calculated when J=0. The results are compared with the available experimental data and other theoretical values. And the comparison shows that excellent agreement exists between the present results and the experimental data.Then, using the same MRCI approach and Dunning's correlation consistent basis sets, the PECs for the GeS (a3Σ+, b3Π, A1Π,13Σ- and 15Σ+) molecule are investigated. The basis set used for Ge and S atoms is aug-cc-pVQZ and aug-cc-pV5Z. The effects on the spectroscopic parameters by the Davidson modification and core-valence correlation and relativistic corrections are also included. The way to consider the relativistic correction is to use the second-order Douglas-Kroll Hamiltonian approximation at the level of cc-pV5Z basis set. The core-valence correlation correction is carried out with the aug-cc-pCVTZ basis set. Fitting the PECs including the Davidson modification and core-valence correlation and relativistic corrections, the spectroscopic parameters and molecular constants are determined. The two-point correlation-energy extrapolation is also employed in computation. In detail, the extrapolation is based on the aug-cc-pVQZ and aug-cc-pV5Z basis sets. Using the PECs obtained by the extrapolation including the Davidson modification and core-valence correlation and relativistic corrections, the corresponding spectroscopic parameters and molecular constants are obtained.On the whole, by comparison with the measurements, the present Davidson modification, core-valence correlation and relativistic corrections and the total-energy extrapolation could obviously improve the quality of spectroscopic parameters.