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 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 (s), dissociation energies (s), excitation energies (s), harmonic vibrational frequencies (s), and anharmonicities (s) 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 () values are compared with the CCSD(T) values. The most stable 1Π (1σ2σ3σ1π) and 1Δ (1σ2σ3σ1π1δ) electronic states of TaB lie close in energy with ∼62 kcal mol with respect to the Ta(F) + B(P) asymptote. However, spin-orbit coupling effects make the 1Δ state the true ground state of TaB. The ground electronic state of WB (1Π) has the 1σ2σ3σ1π1δ electron configuration and is followed by the excited 1Σ and 1Δ states. Finally, the MRCI , , , and values of the 1Π state of TaB and 1Π and 1Δ 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).