CCL: Farewell to Bjorn Roos

 Sent to CCL by: "Christopher  Cramer" [cramer%%umn.edu]
 Today brings the sad news that Professor Bjorn Roos of Lund University has
 passed away. I
 subscribe to the belief that practitioners of science should appreciate not only
 the technical aspects
 of their field, but also the human history that underlies its development. In
 that vein, I hope that
 CCL readers will find the below summary of Bjorn's scientific accomplishments to
 be of interest. His
 insights, and his delicious irascibility, which was always tempered with humor,
 will be sorely missed
 by his many friends and colleagues, and the broader scientific community in
 general.
 I note that his name, Bjorn, should carry a diacritical umlaut on the
 "o", but I have left it off to
 ensure email portability, and somehow "Bjoern" simply doesn't seem
 quite right... In any case:
 Professor Roos' contributions to theoretical chemistry were foundational at the
 most fundamental
 level:  each of his developments opened some broad new area of chemistry that
 had hitherto seen
 little successful theoretical study. Professor Roos' models continue to be put
 to practical use every
 day in modern calculations, and in many cases continue to be the benchmark
 calculations against
 which new model developments are measured.
 Professor Roos began his studies of quantum chemistry just as the community was
 beginning to
 recognize the potential for digital computers to assist in the solution of the
 complex numerical
 problems associated with solution of the Schrdinger equation for molecular
 systems. His first
 major breakthrough was the development of the direct configuration interaction
 (CI) method in
 1972. In a particularly clever insight, Professor Roos recognized that the very
 expensive formation
 and diagonalization of the CI matrix could be replaced by an iterative procedure
 involving only the
 two-electron repulsion integrals already available from an antecedent
 Hartree-Fock (HF)
 calculation. This algorithmic advance reduced the time required for a typical CI
 calculation by one
 to two orders of magnitude for a typical molecular system. Early applications of
 this paradigm-
 shifting approach included the determination of the bond energy in the water
 dimer with an
 accuracy of better than 1 kcal/mol, the identification of hydrogen isocyanide
 (HNC) in  interstellar
 space through comparison to its computed rotational spectrum, and the
 calculation of the energy
 surface for H3 with an accuracy of 0.1 kcal/mol (the latter work involved Roos'
 then student, Per
 Siegbahn, now Professor at Stockholm University).
 Professor Roos' next key development came in 1980, when he and his co-workers
 described the
 complete active space self-consistent field (CASSCF) method. Prior to this
 point, theoretical studies
 had focused predominantly on closed-shell singlets and other electronic states
 readily described by
 single determinants. The CASSCF model, however, was completely general and
 opened the way to
 even-handed treatment of all types of electronic structures, independent of open
 shell character,
 spin multiplicity, etc. Thus, using the CASSCF model, it became possible to
 generate accurate
 potential energy surfaces across regions of bond making and bond breaking (i.e.,
 to study chemical
 reactions) and also to rigorously describe excited molecular electronic states.
 The CASSCF method
 continues to be one of the most important standard tools in quantum chemistry
 for the calculation
 of complex molecular properties in both ground and excited states of molecular
 systems. Indeed, it
 is still the only method which can be used for the general treatment of
 excited-state potential
 energy surfaces (photochemistry) although newly developed models based on
 time-dependent
 density functional theory are beginning to show promise. Professor Roos has
 recently extended the
 CASSCF model to permit spin-orbit coupling between different electronic states.
 The combination
 of this model with a Douglas-Kroll-Hess Hamiltonian (to account for scalar
 relativistic effects) has
 led to an explosion of activity in the application of the CASSCF model to
 lanthanide and actinide
 compounds, including a fascinating paper in Nature in 2005 that addressed the
 unique bonding in
 the U2 dimer and led to a renaissance of interest in the fundamental nature of
 the chemical bond.
 In typical application, the CASSCF method treats only the so-called nondynamical
 part of the
 electron-electron correlation problem. The missing part (dynamical correlation)
 can be treated
 using a multireference (MR) variation of Roos' direct CI, but such a direct MRCI
 can only be
 practically undertaken for very small molecules. Recognizing this limitation,
 Roos contributed yet
 another fundamental methodological advance and developed over the course of the
 1980s a
 multireference perturbation theory for computing the electron correlation
 energy:  CASPT2. The
 CASSCF/CASPT2 model has proven to be remarkably accurate and robust, making it
 the model of
 choice for the study of many reactions and, particularly, for the study of
 photochemistry and
 excited-state reactivity. In the absence of an experimental measurement, the
 typical benchmark for
 an excited-state energy would be a CASPT2 calculation. One measure of the
 utility and popularity
 of the CASSCF/CASPT2 model is the degree to which it has been coded into the
 most widely used
 electronic structure program suites, including the one developed by Professor
 Roos himself,
 MOLCAS, which has a user base of hundreds of research groups.
 Professor Roos' approach to quantum chemistry was always to develop methods
 which could be
 used in large-scale applications to real chemical problems. He and his
 co-workers demonstrated
 the scope of his methodologies in pioneering applications, e.g., characterizing
 the spectroscopic
 properties of the blue copper proteins (which led to a new understanding of the
 concept of strain in
 biochemistry), describing the electronic spectra of numerous organic molecules,
 including the DNA
 and amino acid chromophores, and studying the chemical bonds in transition-metal
 compounds
 (to include the only successful description to date of the chemical bond in
 Cr2). As noted above,
 particularly recent applications have focused on the heaviest elements in the
 periodic table,
 including studies of the dynamics of the uranyl ion in water, and the
 characterization of complexes
 of uranyl and neptunyl with carbonate and water (relevant to the reprocessing of
 spent nuclear
 fuel). Key to several of these studies ws also Roos' development of especially
 well balanced basis
 sets for all-electron calculations, and recent and important developments using
 Cholesky
 decomposition to speed integral evaluation, (the latter area involving Roos'
 long-time co-worker
 Roland Lindh, Lecturer at Lund and now Professor at Uppsala).
 The methodological developments pioneered by Professor Roos dramatically and
 uniquely advanced
 wave-function theories in quantum chemistry. The influence of the direct CI,
 CASSCF, and CASPT2
 models on modern quantum chemistry have been profound, particularly in the
 degree to which they
 have permitted the extension of accurate theoretical models to enormous swaths
 of the periodic
 table that had been previously inaccessible. This is a testament to the
 creativity and vision of
 Professor Roos, who repeatedly combined an appreciation for chemical relevance
 with brilliant
 physical and algorithmic insights.
 In San Francisco, at the upcoming spring National Meeting of the American
 Chemical Society, Bjorn
 was to have received the ACS Award in Theoretical Chemistry. Roland Lindh and
 Laura Gagliardi will
 be presenting on his behalf during the award symposium in the Division of
 Physical Chemistry. I
 have no doubt that attendees who knew him better than I will be able to share
 more stories of
 Bjorn, including especially those that go beyond his prodigious scientific
 accomplishments, and I
 look forward to the chance to learn more about him there.
 Respectfully,
 Chris Cramer
 University of Minnesota