Theoretical Investigation of the Electronic States of Calcium Carbide, CaC

Abstract

Using multireference methods and semiquantitative basis sets, we have computed nineteen potential energy curves and spectroscopic parameters of the calcium monocarbide system, CaC. For the much debated 3Σ- and 5Σ- states our calculations definitively show that the ground state is of 3Σ- symmetry with the a5Σ- state lying 1500−2500 cm-1 higher. For the X3Σ- state our best estimates for the binding energy and equilibrium bond length are 51 kcal/mol and 2.33 Å, respectively. All states are strongly ionic with a charge transfer of 0.5 to 0.7 e- from Ca to C.

Ground states of BeC and MgC: A comparative multireference Brillouin–Wigner coupled cluster and configuration interaction study

“…The MRBW-CCSD results agreed with large MR-CI calculations predicting a 3 ⌺ Ϫ ground state for CaC [8]. While our paper was in press, an experimental study by Ziurys and coworkers [10] in which the pure rotational spectrum of the CaC radical was recorded in the laboratory confirmed the X 3 ⌺ Ϫ identity of its ground state.…”

“…The corresponding numbers for the 5 ⌺ Ϫ state are 0.43 and 0.61 electrons. The same kind of bonding was observed in the isovalent molecules CaC and ZnC and for the same symmetries [7][8][9].…”

“…For the C atom specifically, this set was augmented by a set of diffuse functions, aug-cc-pVQZ [22b]. This augmentation was deemed necessary, because carbon was observed to acquire significant negative charge as in the isovalent CaC and ZnC molecules [7][8][9]. In order to assess the adequacy of the basis set and to eliminate any relevant superposition errors, we have also performed calculations with the aforementioned methods near the equilibrium bond distance in both X 3 ⌺ Ϫ and 5 ⌺ Ϫ states of BeC and MgC, employing the cc-pVnZ (Be, Mg) /aug-cc-pVnZ (C) series, with n ϭ T(3), Q(4), and 5, and then extrapolating the binding energies D e and bond lengths r e to the complete basis set (CBS) limit, using the simple exponential formula [23]:…”

“…The reason lies behind the (ns, np) "near-degeneracy" of the M atomic orbitals, resulting in a multireference wavefunction with significant biexcited ns 2 3 np 2 character, even at the equilibrium geometry. In a recent paper, we employed the MRBW-CCSD method in CaC and ZnC in order to compare its performance against conventional single-reference and multireference methods [8,9]. In the most interesting case of CaC, upon inclusion of the "semicore" 2s 2 2p 6 electron correlation of Ca, the CCSD and CCSD(T) methods predicted a 5 …”

“…The reason single-reference methods tend to underestimate the 5 ⌺ Ϫ 4 3 ⌺ Ϫ gap in alkaline earth metal (and the other isovalent) carbides, leading even to a ground-state character change, for example, in BeC and CaC (see above), can be traced to the nature of these competing states [7,8]. The quintet state originates from the M(…”