Research in Molecular and Biomolecular Magnetism
Rodriguez Research Group
Transition metals, such as iron and manganese, play a crucial role in the physical, chemical, and magnetic properties of molecular systems of interest in a wide variety of fields. For example, open-shell iron ions, which have unpaired electrons in their partially filled valence shells, are at the heart of the functional mechanisms of many proteins and of the fascinating magnetic properties of some mesoscopic nanostructures.
The net spin associated with metal-centered unpaired electrons gives rise to a number of spectroscopic and magnetic properties which are studied in the field of "molecular magnetism". Whereas some magnetic properties can be described in the non-relativistic limit, some others can only be understood by explicit inclusion of relativistic effects. In particular, by inclusion of spin-orbit coupling.
Properties associated with relativistic effects, such as spin-orbit coupling (SOC), are among the most difficult to predict from first principle (ab-initio) electronic structure calculations. In transition metal-containing systems, such as metallo-proteins, bioinorganic complexes, and molecular nanomagnets, valence shell electrons of different constituent atoms interact with each other not only via their intrinsic charge but, also, via their spin degrees of freedom. In particular, each charged electron's orbital motion couples with the intrinsic spin of all other electrons giving rise to a multitude of electron-electron SOC interactions. In computational terms, such interactions give rise to thousands and thousands of two-electron integrals with a concomitantly large, and often huge, computational cost.
These calculations require the use of supercomputers or clusters of parallel-processing home computers. Therefore, it is of interest to implement ab-initio methodologies which display, simultaneously, high accuracy and computational efficiency.
Conventional, i.e., non-relativistic, electronic structure calculations based on spin density functional theory (SDFT) already take into account electron-nuclear and electron-electron interactions associated with most classical and non-classical effects. SOC however, being a relativistic effect, is not incorporated in conventional SDFT calculations. For molecular systems, where SOC can be considered a numerically small but important perturbation to the non-relativistic molecular electronic Hamiltonian, it is natural to think of perturbation theory (PT) as a means of incorporating such relativistic effect. Such is the computational approach implemented at Purdue. Namely, a second-order perturbation treatment of SOC on top of non-relativistic SDFT calculations: SDFT-PT.
After several years of computational implementation, testing, and validation,
the Theoretical and Computational (Bio)molecular Physics Group, led by Prof. Jorge Rodriguez, has implemented an accurate computational methodology for predicting some effects of spin-orbit coupling (SOC), on physico-chemical properties of systems as diverse as bioinorganic complexes, including metallo-proteins, and metal-containing single molecule magnets (SMM). In particular, a phenomenon known in the field of molecular magnetism as "zero field splitting" (ZFS), which removes the ground state spin-degeneracy of a system, and the related "magnetic anisotropy" can be predicted with good accuracy. Recent Ph.D. recipient Dr. Fredy Aquino and Prof. Rodriguez published a paper [J. Phys. Chem. A, Vol. 113, 9150, 2009] describing a methodology, based on the combination of spin density functional theory and perturbation theory (SDFT-PT), which is capable of predicting the ZFS of transition metal-containing complexes of relevance to metallo-enzyme biochemistry, inorganic/bioinorganic chemistry, and molecular magnet-based nanotechnology. Their latest paper follows a previous paper on the subject [J. Chem. Phys., Vol. 123, 204902, 2005].
The Purdue SDFT-PT implementation illustrates the power of computational methods of electronic structure, such as conventional ab-initio and spin density functional theory, for the prediction and microscopic interpretation of spin-dependent properties of metal-containing systems where subtle, but important, electron-electron correlations are present.
This research was made possible by funding from the National Science Foundation, NSF, via a CAREER award to Prof. Rodriguez.