Electronic structure and spin coupling of the manganese dimer: The state of the art of ab initio approach

Abstract

A thorough ab initio study of the Mn(2) dimer in its lowest electronic states that correlate to the ground Mn((6)S)+Mn((6)S) dissociation limit is reported. Performance of multireference methods is examined in calculations of the fully spin-polarized S=5((11) summation operator(+) (u)) state against the recent accurate single-reference coupled cluster CCSD(T) results [A. A. Buchachenko, Chem. Phys. Lett. 459, 73 (2008)]. The detailed comparison reveals a serious disagreement between the multireference configuration interaction (MRCI) and related nonperturbative results on the one hand and the complete active space perturbation theory (CASPT) calculations on the other. A striking difference found in the CASPT results of the second and third orders indicates poor perturbation expansion convergence. It is shown that a similar problem has affected most of the previous calculations performed using CASPT2 and similar perturbative approximations. The composition of the active space in the reference multiconfigurational self-consistent field calculations, the core correlation contribution, and basis set saturation effects are also analyzed. The lower spin states, S=0-4, are investigated using the MRCI method. The results indicate a similar dispersion binding for all the spin states within the manifold related to the closed 4s shells, which appears to screen and suppress the spin coupling between the half-filled 3d atomic shells. On this premise, the full set of model potentials is built by combining the accurate reference CCSD(T) interaction potential for S=5 and the MRCI spin-exchange energies for the S<5 states. This approach leads to the value of 550 cm(-1) as a lower bound for the (1) summation (+) (g) ground-state dissociation energy. The spin-exchange energies themselves are found to comply with the simple Heisenberg model. The effective spin-coupling parameter J is estimated as -3.9 cm(-1), a value roughly 2.5 times smaller in magnitude than those measured in the inert gas cryogenic matrices. Compressing of the Mn(2) dimer in the matrix cage is suggested as the prime cause of this disagreement.

Cold magnetically trapped2Dgscandium atoms. I. Interaction potential

“…Notably, the bond lengths are consistently shorter at the NEVPT2 level of theory. This is in agreement with the findings of Buchachenko et al [25], who found perturbation approaches generally give too short bond lengths and overestimate the well depth for transition metal dimers.…”

“…The active space contains six electrons in symmetry adapted combinations of the 4s and 3d valence, and the 4p virtual atomic orbitals. This choice of active space, termed full valence CAS, was found preferable in earlier work on transition metal dimers [23,25]. Subsequently, single-state NEVPT2 calculations were performed.…”

“…In this paper we therefore employ the single-state NEVPT2 method [27][28][29][30][31]. This method was found to perform remarkably better than other perturbation theories, for the manganese dimer [25]. It provides a properly convergent perturbation expansion, as it is intruder-state free.…”

“…The applicability of various multireference (MR) approaches to the electronic structure of transition metal dimers was assessed for manganese by Buchachenko et al [25]. These methods are MR single and double excitation configuration interaction (MRCI), with Davidson correction for higher excitations (MRCI+Q), averaged quadratic coupled cluster (AQCC), and the second-and third-order perturbation theories: complete active space perturbation theory (CASPTn), n-electron valence state perturbation theory (NEVPTn), multiconfigurational quasidegenerate perturbation theory (MCQDPTn), and multireference Møller-Plesset perturbation theory (MRMPn), n = 2,3.…”

Direct observation of high-spin states in manganese dimer and trimer cations by x-ray magnetic circular dichroism spectroscopy in an ion trap

“…A similarly complex behavior can also be found in manganese molecules and clusters. 2,3 The neutral Mn 2 molecule has been extensively studied by theory [4][5][6][7][8][9][10][11][12][13] and is characterized by a 1 Σ + g ground state in agreement with electron spin resonance 14 as well as resonance Raman and visible absorption 15 spectroscopy studies of matrix isolated Mn 2 . Most theoretical studies agree on nearly degenerate 2S = 0 to 2S = 10 states 4-13 that render the description of Mn 2 computationally challenging.…”