Wavefunction theory and density functional theory analysis of ground and excited electronic states of TaB and WB

Several low-lying electronic states of TaB and WB molecules were studied using ab initio multireference configuration interaction (MRCI), Davidson corrected MRCI (MRCI+Q), and coupled cluster singles doubles and perturbative triples [CCSD(T)] methods. Their full potential energy curves (PECs), equilibrium electron configurations, equilibrium bond distances (r_es), dissociation energies (D_es), excitation energies (T_es), harmonic vibrational frequencies (ω_es), and anharmonicities (ω_ex_es) are reported. The MRCI dipole moment curves (DMCs) of the first 5 electronic states of both TaB and WB are also reported and the equilibrium dipole moment (μ) values are compared with the CCSD(T) μ values. The most stable 1³Π (1σ²2σ²3σ¹1π³) and 1⁵Δ (1σ²2σ²3σ¹1π²1δ¹) electronic states of TaB lie close in energy with ∼62 kcal mol^-1D_e with respect to the Ta(⁴F) + B(²P) asymptote. However, spin-orbit coupling effects make the 1⁵Δ₀ state the true ground state of TaB. The ground electronic state of WB (1⁶Π) has the 1σ²2σ¹3σ¹1π³1δ² electron configuration and is followed by the excited 1⁶Σ and 1⁴Δ states. Finally, the MRCI D_e, r_e, ω_e, and ω_ex_e values of the 1³Π state of TaB and 1⁶Π and 1⁴Δ states of WB are used to assess the density functional theory (DFT) errors on a series of exchange-correlation functionals that span multiple-rungs of the Jacob's ladder of density functional approximations (DFA).