Theoretical and Structural Chemistry. As suggested by our logo, we cover a large thematic area. (Octahedron stands for inorganic and coordination branches, hexagon represents the organic chemistry, the icosahedron hints the clusters and organometallics; the bi-lobal shape and torus signify the specialized issue of magnetic anisotropy). Such aperture is possible since, in spite of specific differences the bond mechanisms are unitary. In other word, all compounds and materials show structures and properties determined by quantum chemistry.

Group Members:

Dr. Fanica Cimpoesu

Dr. Ana Maria Toader

Dr. Maria Cristina Buta

Dr. Bogdan Frecus

Bond. Chemical Bond!

The group was constituted in 2016. We deny ourselves the facile routine works. We do computation with state-of-the-art methods (such as DFT and CASSCF) but also enjoy mastering less visited avenues, such is nowadays the Valence Bond (VB), the very first theory of chemical bond.

We rely much on phenomenological (effective) Hamiltonians. Used as post-computational tools, these are illuminating the content of black-box of big calculations and establish a common language with experimental chemistry). To work this, we write our own codes, in matlab, mathematica, fortran. Aside computers, we use pencil, paper and gum-erasers.

To put the above statement in correct light, we say that in 50 years from now the DFT methods will no longer resist in any of the actual forms (a lot of non-stransparent empirical ingredients), while phenomenological approaches like Ligand Field theory, or Heisenberg Exchange Hamiltonian will remain ever-green. They are still in use nowadays, when reach almost a century, and will resist over ages, being yet empirical but transparent.

At the confluence of specialized ab-initio calculations and skillful use of effective Hamiltonian, we note our interests and capabilities in the vibronic (vibrational-electronic) interactions, i.e. going one step beyond Born-Oppenheimer approximation of static nuclei.

We built a strong-hold in the quantum chemistry of lanthanide compounds. Not many groups can do this, because implies a good chemical intuition and advanced handling in calculation setting and post-computational analysis. The lanthanides complexes can be characterized as non-aufbau structure, i.e. a situation not implemented in current quantum codes.

Transition metal coordination complexes are also in our focus. Common with lanthanide chemistry, we have innovative procedures in treating the magnetic anisotropy. Magneto-structural correlation is a fascinating paradigm, to which we brought new interesting case studies with transition metal and lanthanide complexes.

Magneto-structural correlation with non-metal spin carriers is a domain where we brought interesting breakthroughs, working systematic analyses on stable nitroxide-based organic radicals and the special class of triangulenes (aromatic hydrocarbons with regular triangular shapes, carrying spin by topological reasons). This area is not much organized in the spirit of overlap-driven balance of ferro-antiferro coupling, here claiming pioneering realizations

The spin biochemistry. Since the spin interactions are essential in life-chemistry, in respiratory systems and anti-oxidant activity, we are currently initiating a front of work in this domain too.

Back to the basis!

The most audacious activity of the group, still remained hidden in the background of tedious technical preparative is the revisiting of the actual Gaussian Type Orbitals (GTO) infrastructure, having deeper flaws than commonly acknowledged by most users. For details visit the corresponding entry on this site.

Divagation. Our philosophy.

The classical-modern view on philosophy of science, originates from yearly views of K. Popper, stating the principle of falsiability [Karl Popper, The Logic of Scientific Discovery, Hutchinson & Co., London, 1959 (1st ed. Springer, 1935)], amended by T. Kuhn, observing the quasi-independence of so-called paradigms (sort of axiomatic-alike bodies for a given set of problems) and normal science as puzzle solving, which fails after certain periods of stagnation in routine growth, determining steps forwards [Thomas S. Kuhn, The Structure of Scientific Revolutions, University of Chicago Press, Chicago, 1962]

We appreciate the actual status of quantum chemistry as a on the route of approaching a cliff, after a long plateau of stagnation (in Kuhns sense), determined by the extensive use of imperfect tools. Approximations like Gaussian-Type-Orbitals (GTOs) where welcomed and necessary at the dawn of quantum chemistry, when computers were weaker than a nowadays portable phone and of the size of a big room. As a detail, note that not only the exp(-ar²) components form an approximation, but GTOs are staying under a severely overlooked limitation (most users are not aware, the developers are afraid on the cost of reshaping the status-quo).

Namely, in all existing GTO-based codes (the quasi-totality of available quantum chemistry tools) the s-type orbitals are composed only by exp(-ar²) primitives, the p-type ones only by r^.exp(-ar²), the d-shell accounting only by r²exp(-ar²), and so on. It will be reasonable and technically more satisfying to allow s-orbitals being termed by a larger degree of freedom, like the{exp(-ar²), r^.exp(-ar²), r²exp(-ar²), etc} sets, the p-orbitals to be worked with { r^.exp(-ar²), r²exp(-ar²), r³exp(-ar²), etc} primitives, enabling { r²exp(-ar²), r³exp(-ar²), r⁴exp(-ar²), etc} the d-orbitals, and so on. To introduce this apparently small upgrading implies rewriting the most of core-routines in existing codes and replace all the existing GTO repositories, i.e. a painful infrastructural change, however perfectly feasible and desirable, nowadays. In meanwhile, with artificially constrained GTO patterns, we are invited to use bigger and bigger basis sets, to acquire precision which otherwise could be easier attained within the suggested scheme of more flexible GTOs. Such problems may not exist in the world of plane-wave bases, mostly used by physicist, but there are there also pitfalls from approximation.

Another factor of stagnation may be the vogue of Density Functional Theory (DFT), correctly seen, as matter of principle, as cheaper technical alternative for a many problems, otherwise resolved in a more demanding way in the frame of Wave Function Theory (WFT). After all, density is an intuitive and measurable object, while the wave function interpretation gave headaches since the beginning of quantum theory. The problem is that the Phylosophers Stone and Graal of DFT, the exact definition of the Functional, is not known, appearing, for the sake of practical application, a large zoo of empirical functionals. The problem is further pushed on the quick-sand terrain of empirical ingredients by growing a full garden of long-range corrections, to cure intrinsic drawbacks of DFT at outskirts of atoms and in the approach of supra-molecular assemblies.

We bet that in a half-century from now, or even much quicker, the DFT, as we know it, will no longer exist, since larger computers will enable encompassing actual WFT bottlenecks. On the other hand, DFT deserves be kept in the panoply of quantum methods because of its conceptual virtues, such as absolute definition of electronegativity, chemical hardness and related acid-base affinity scales. From this perspective, we position ourselves as strong critics of practical DFT (particularly about certain wrong applications, beyond validity of DFT funding theorems, e.g. in the quasi-degenerate circumstances of lanthanide compounds), while we declare ourselves fans of conceptual DFT, indulging, for its virtues, the actual limitations.

Although DFT was tacitly thought as a tool of technical shortcut, circumventing the WFT hardships, the true perennial entities in the quantum chemistry are the phenomenological models, with their effective Hamiltonians. In spite of being empirical constructs, the simplified parametric scheme serves to gain insight in the problem. These models represent the right tools to illuminate black-box of heavy numeric calculations and the lingua franca for communication with experimental chemists and their instrumental data. Phenomenological models like Ligand Field Theory, Heisenberg Spin Hamiltonian are still in use nowadays, and will remain evergreens over years and decades.

Phenomenological Hamiltonians should not be regarded with condescendence, as obsolete. Actually, the peoples able to master them properly (beyond the qualitative generalities taught in in general Curriculum of chemists and physicists) are remaining fewer and fewer, under the hype of high-precision and big data. The algebraic Ligand Field technicalities are quite complex, the Heisenberg effective Hamiltonian matrices, possibly reaching the size of millions of microstates, can challenge computer resources much more that a DFT calculation of large molecular and supramolecular systems.

Indulging ourselves a parallel between accuracy of image rendering in art and the trend to numeric precision in quantum chemistry (see the above composition), we would predict that, similarly to the art itself, the state-of-the-art in quantum chemistry will drop the current longing for high precision, when the computers will enable this as a not so special goal. We aim to work for such desiderata.

The main assets in a quantum chemistry group are the brains of scientists, not the power of HPC stations at hand.

We would add our personal perspective, as new continuation. after the Poppers and Kuhns line. We observe that science and scientist are, as size, only a small crust of society. Therefore, as structure, the science borrows organization patterns already known in larger social shells. It does this with a certain timeline delay. Thus, science initially it borrowed infrastructure models from the Church. The universities were initially organized copying catholic institutions. Later on, in the positivist era of XIX-th century, the scientists, as casts organized by meritocracy, borrowed honor codes and protocols from former aristocracy (while aristocracy itself, as class, was by then, in twilight dusk). Nowadays, with a certain of delay, the science borrows forms from industrialization era. Groups are growing bigger and aim for productivity, quantitatively and severely measured. Professors, instead of noble bearers of enlightenment, are valued merely as CEO and PR agents. Pushed by indices with somewhat artificial construct, scientists write more than can read. There is little inclination for the curtesy of mutual acknowledgement by intrinsic values of their beings and realizations.

As anecdotical issue, note that, during his life, Einstein got a Hirsh index 27.

Running for output, artificial production is encouraged, e.g. reenacting with a bigger basis sets and newer functionals a calculations done one-two decades ago. Aiming, while often just pretending, to give results with applicational relevance, peoples hurry to grasp with improper and confusing tools complex subjects, which otherwise would deserve a longer maturation time for a better methodological frame. Because of the race for quantitative scientific outcome, science will go, if not already there, into a sort of supra-production crisis, as went till now industrialized society several times till now. Simply because, as pointed, science borrows organization and goals from this frame and shares the same functioning traps.

As working philosophy, being caught between generations (except one younger actual group member), we do not appreciate very much the mirages of a coming brave new world, seeing in turn what deserves to be conserved from good old times. Thus, our aim is to quality instead of quantity. Some of our group members are old enough to confess that started in difficult conditions and only after 2000 the path cleared sufficiently for a more-or-less normal carrier. Not only because of a starting handicap in acquiring quantitative size to our CVs (large number of articles, high Hirsch, fighting for visibility and citation), as matter of credo and strategy, we do not aim for a large mass of CV, striving for sharp-good quality and emulative innovation. Our papers are proving this claim. We still advocate for quality as good measure of nominal curriculum vitae achievements (i.e. literally, the course of life as real flow), accomplishing as much as possible the actual demands in good CV indices.