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 e s), dissociation energies ( D e s), excitation energies ( T e s), harmonic vibrational frequencies ( ω e s), and anharmonicities ( ω e x e 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 ( μ ) values are compared with the CCSD(T) μ values. The most stable 1 3 Π (1σ 2 2σ 2 3σ 1 1π 3 ) and 1 5 Δ (1σ 2 2σ 2 3σ 1 1π 2 1δ 1 ) electronic states of TaB lie close in energy with ∼62 kcal mol -1 D e with respect to the Ta( 4 F) + B( 2 P) asymptote. However, spin-orbit coupling effects make the 1 5 Δ 0 + state the true ground state of TaB. The ground electronic state of WB (1 6 Π) has the 1σ 2 2σ 1 3σ 1 1π 3 1δ 2 electron configuration and is followed by the excited 1 6 Σ + and 1 4 Δ states. Finally, the MRCI D e , r e , ω e , and ω e x e values of the 1 3 Π state of TaB and 1 6 Π and 1 4 Δ 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).