On the ground states of CaC and ZnC: A multireference Brillouin–Wigner coupled cluster study

Abstract

We test the recently developed state-specific multireference Brillouin-Wigner coupled cluster ͑MRBWCCSD͒ method against the single reference CCSD method by examining theoretically the competing X 3 ⌺ Ϫ and 5 ⌺ Ϫ states of the ͑experimentally unknown͒ isovalent calcium and zinc carbide diatomics ͑CaC, ZnC͒. At the CCSD level, CaC is ''incorrectly'' predicted to have a ground 5 ⌺ Ϫ state; however, the MRBWCCSD treatment restores the correct state ordering, and improves significantly the energetics for both molecules. Further comparison with various single-and multireference treatments shows that the latter are absolutely necessary for obtaining meaningful results for the ground states in both molecules.

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

“…Ϫ state by at least 10 kcal/mol (CCSD level) [7]. The MRBW-CCSD results agreed with large MR-CI calculations predicting a 3 ⌺ Ϫ ground state for CaC [8].…”

“…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 extra correlation energy gained by the (T) noniterative correction is sometimes not sufficient to cure the lack of "nondynamical" correlation energy. Recent tests to such cases using a multireference coupled cluster approach [5] based on the use of the Brillouin-Wigner resolvent (MRBW-CCSD) have been successful so far [6,7]. The diatomic alkaline-earth metal carbide (MC) diatomics and the isovalent carbides of group 12 of the periodic table have proven to be a good testing ground for multireference methods [7].…”

“…Recent tests to such cases using a multireference coupled cluster approach [5] based on the use of the Brillouin-Wigner resolvent (MRBW-CCSD) have been successful so far [6,7]. The diatomic alkaline-earth metal carbide (MC) diatomics and the isovalent carbides of group 12 of the periodic table have proven to be a good testing ground for multireference methods [7]. 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.…”

“…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(…”