Multiconfigurational electronic structure theory calculations including spin-orbit coupling effects were performed on four uranium-based single-molecule-magnets. Several quartet and doublet states were computed and the energy gaps between spin-orbit states were then used to determine magnetic susceptibility curves. Trends in experimental magnetic susceptibility curves were well reproduced by the calculations, and key factors affecting performance were identified.
The magnetic coupling in transition metal compounds with more than one unpaired electron per magnetic center has been studied with multiconfigurational perturbation theory. The usual shortcomings of these methodologies (severe underestimation of the magnetic coupling) have been overcome by describing the Slater determinants with a set of molecular orbitals that maximally resemble the natural orbitals of a high-level multiconfigurational reference configuration interaction calculation. These orbitals have significant delocalization tails onto the bridging ligands and largely increase the coupling strengths in the perturbative calculation.
Density functional theory, Complete Active Space Self-Consistent Field (CASSCF) and perturbation theory (CASPT2) methodologies have been used to explore the electronic structure of a series of trichromium Extended Metal Atom Chains (EMACS) with different capping ligands. The study is motivated by the very different structural properties of these systems observed in X-ray experiments: the CN-capped example has a symmetric Cr unit while for the NO-capped analogue the same unit has two very different Cr-Cr bond lengths. Density functional theory fails to locate an unsymmetric minimum for any of the systems studied, although the surface corresponding to the asymmetric stretch is very flat. CASPT2, in contrast, does locate an unsymmetric minimum only for the NO-capped system, although again the surface is very flat. We use the Generalized active space (GASSCF) technique and effective Hamiltonian theory to interrogate the multi-configurational wavefunctions of these systems, and show that the increase in the σ-σ* separation as the chain becomes unsymmetric plays a defining role in the stability of the ground state and its expansion in terms of configuration state functions.
Extended metal atom chains constitute an interesting class of molecules from both theoretical and applied points of view. In the chromium-based series Cr 2 M(dpa) 4 X 2 (with M = Zn, Ni, Fe, Mn, Cr), the direct metal−metal interactions span a wide range of possibilities and so do their associated properties. The multiplicity and symmetry components of the metal−metal bond are herein analyzed via the effective bond order (EBO) concept using complete active space self-consistent field wave functions and compared with similar bimetallic Cr 2 L 4 X 2 systems. The bond multiplicity follows a trend dominated by the Cr−Cr distance which, in turn, depends on the nature of the axial ligand (X). Cr 2 M compounds present asymmetric structures with virtually no interaction between the Cr 2 unit and M, whereas fully symmetric structures with delocalized bonding among the three metals are also possible in Cr 3 complexes. In such cases, a strategy that involves localization of the molecular orbitals into each Cr−Cr pair is applied to quantify the contribution of each pair to the overall metal−metal bond multiplicity.
Both density functional theory and multi-configurational ab initio (CASPT2) calculations are used to explore the potential energy surface of the hexagonal prismatic cluster [Mn@Si12](+). Unlike isoelectronic Cr@Si12, the ground state is a biradical, with triplet and open-shell singlet states lying very close in energy. The results are discussed in the context of recent experimental studies using infra-red multiple photon dissociation spectroscopy and X-ray MCD spectroscopy.
Recently published static DFT and CASSCF/CASPT2 calculations depicted extremely flat Potential Energy Surfaces (PESs) for the Cr-Cr flexibility of Cr(dpa)X (X = NCS, CN, NO) extended metal atom chains (EMACs) (M. Spivak, et al., Dalton Trans., 2017, 46, 6202). We herein explore the thermal and crystal packing effects on the structure of EMACs using ab initio molecular dynamics (MD). Car-Parrinello DFT-based simulations of the isolated molecules show that thermal energy favors asymmetric arrangements of the Cr chain due, in part, to the bending of the axial ligands (X) and the increased X-Cr distance, both of which weaken X → Cr σ-donation. This effect is even more prominent in the crystalline phase due to the interaction between the axial ligands of neighboring molecules in the unit cell. This could explain the typical discrepancies between the experimental and theoretical characterization of Cr EMACs observed in the literature.
Modeling and simulation of small molecules such as drugs and biological cofactors have been both a major focus of computational chemistry for decades and a growing need among computational biophysicists who seek to investigate the interaction of different types of ligands with biomolecules. Of particular interest in this regard are quantum mechanical (QM) calculations that are used to more accurately describe such small molecules, which can be of heterogeneous structures and chemistry, either in purely QM calculations or in hybrid QM/molecular mechanics (MM) simulations. QM programs are also used to develop MM force field parameters for small molecules to be used along with established force fields for biomolecules in classical simulations. With this growing need in mind, here we report a set of software tools developed and closely integrated within the broadly used molecular visualization/analysis program, VMD, that allow the user to construct, modify, and parametrize small molecules and prepare them for QM, hybrid QM/MM, or classical simulations. The tools also provide interactive analysis and visualization capabilities in an easy-to-use and integrated environment. In this paper, we briefly report on these tools and their major features and capabilities, along with examples of how they can facilitate molecular research in computational biophysics that might be otherwise prohibitively complex.
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