<i>Ab-initio</i> MRD-CI Studies of electric field gradient (efg) and dipole moment of CCN radical at CASSCF optimized geometries for X²II, a⁴Σ^-, A²Δ, B² Σ^-and C² Σ⁺ electronic states

Abstract

X²II, a⁴Σ^-, A²Δ, B² Σ^-and C² Σ⁺ states of CCN radical have been studied using complete active space self-consistent field (CASSCF), multi-reference singles and doubles configuration interaction (MRD-CI) methods. The optimized geometry for electronic states under consideration seems to be sensitive towards the choice of active orbitals in CASSCF calculations. The equilibrium excitation energies obtained from extrapolated energies in MRD-CI calculation are in excellent agreement with experimental results for X²II<span style="font-size:14.0pt;font-family:Arial;mso-bidi-font-family:" times="" new="" roman";="" color:black"="" lang="EN-IN"> → A²Δ, and

X²II<span style="font-size:14.0pt;font-family:Arial;mso-bidi-font-family:" times="" new="" roman";="" color:black"="" lang="EN-IN"> → C² Σ⁺ systems. The equilibrium excitation energy (T_c), electric field gradient (efg), and dipole moment are sensitive towards the selection of reference configurations, number of roots treated simultaneously, threshold value (T) for configuration selection and quality of starting one-particle basis molecular orbitals in MRD-CI calculations. Natural orbitals (NO's) obtained from iterative natural orbital procedure (I NO) are very good starting one-particle molecular orbitals for reliable prediction of one-electron properties in

MRD-CI calculations.

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