
Björn Olof ROOS
(19372010)
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
twoelectron repulsion integrals already available from an antecedent HartreeFock (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 coworkers described the
complete active space selfconsistent field (CASSCF) method. Prior to this point, theoretical studies
had focused predominantly on closedshell singlets and other electronic states readily described by
single determinants. The CASSCF model, however, was completely general and opened the way to
evenhanded 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 excitedstate potential
energy surfaces (photochemistry) although newly developed models based on timedependent
density functional theory are beginning to show promise. Professor Roos has recently extended the
CASSCF model to permit spinorbit coupling between different electronic states. The combination
of this model with a DouglasKrollHess 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 socalled nondynamical part of the
electronelectron 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
excitedstate reactivity. In the absence of an experimental measurement, the typical benchmark for
an excitedstate 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 largescale applications to real chemical problems. He and his coworkers 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 transitionmetal 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 allelectron calculations, and recent and important developments using Cholesky
decomposition to speed integral evaluation.
The methodological developments pioneered by Professor Roos dramatically and uniquely advanced
wavefunction 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.

