Research interests Last update: 18 July 2001

(In Shinto tradition, the Japanese torii gate indicates that you have come to a sacred place. The sacred nature of the place may be indicated in no other way than by the torii gate. The picture shows a particularly famous gate, accessible only by boat.)

Mark E. Casida Research Interests

• Theoretical chemistry; electronic structure.
• Molecular optical and response properties, including electronic spectra, excited states, dynamic polarizabilities and hyperpolarizabilities, NMR chemical shifts, etc.
• Photochemistry.
• Optical properties of materials.
• Density-functional theory (DFT), especially time-dependent DFT.
• Ab initio.
• Many-body theory.
• Use of many-body Green-function methods to extend the capabilities of DFT.
• DFT exchange-correlation functionals.
• Multi-configurational treatments in many-body methods and in DFT, DFT multiplet problem.
• Auxiliary-function techniques and pseudospectral methods.

Some recent publications.

Some invited talks.

IMPROVING AND EXTENDING TIME-DEPENDENT DENSITY-FUNCTIONAL THEORY METHODOLOGY FOR PHOTOCHEMISTRY AND OPTICAL MATERIALS APPLICATIONS

My research program focuses on improving the understanding of, and ability to predict, molecular electronic spectra, excited states, and photochemistry. This is important for applications in several fields. One such area is the ``molecular engineering'' of optical materials, by designing key molecular constituents to yield desired optical properties. Here the ability to understand and predict the specific properties involved is central. For example, many transition metal coordination compounds undergo changes in color and/or spin state when heated or exposed to light of an appropriate wavelength, and there is considerable interest in exploiting this property to develop ``molecular switches,'' for use in, for example, fast readable-writable optical disks or displays. The problem is to design ligands to yield the desired characteristics, which are intimately related to the spectrum and excited states of the compound.

Due partly to the size of the molecules in question and partly to the fact that the quantum chemistry of excited states is more complicated than that of ground states, the understanding of the spectra, excited states, and photochemistry of molecules of practical interest is still based largely on simple models (both simple theoretical models applied to real systems and high quality ab initio calculations on simpler ``model'' molecules.) This situation is starting to change, due to advances in theoretical methods and algorithms and to improvements in computer technology. Density-functional theory (DFT) has made an important contribution in this regard, for ground state calculations. Several years ago, I developed a molecular formulation of the time-dependent extension of DFT (TDDFT), thus making possible a practical and well-founded density-functional treatment of molecular electronic excitations. (See below.) This method has a lower computational cost than the least expensive of the conventional ab initio methods for treating electronic excitations, yet gives substantially better results. My method has been rapidly adopted, and has now been implemented in most important quantum chemistry programs, that include DFT, worldwide. It is widely used and applications using the method include studies of the spectra of chlorophyll a, fullerenes, polyacetylenes, transition metal coordination compounds, and phototoxic drugs.

While my TDDFT method has met with considerable success, and I will be applying it to optical materials problems, there are still limitations that need to be addressed in order to realize the full power of the method. This is because, although TDDFT is formally exact in the limit of the exact exchange-correlation functional, this unknown functional must be approximated in practice. The approximations used restrict the domain of applicability of the method. Thus the approximate functionals need to be improved and/or ways need to be found to circumvent the difficulties that arise.

For these reasons, my strategy with TDDFT is to apply it to practical problems where applications fall within the known range of validity of TDDFT, while simultaneously working on improving the basic method to expand its range of applicabiity.

Some research accomplishements.

Should you encounter difficulties with these web pages, please contact me at Mark.Casida@UMontreal.CA.