1. Molecular Properties, Structures and Reactivity

Properties are important entities that connect experiment with theory. What we measure is a property, and if we can simulate the property well we can compare the outcome with experiment in order to either qualify the theory or to establish the value of some parameters of interest associated to the measured system. A well qualified theory can then be used to predict and the established parameter values can be used to interpret, these are the most important goals for theory and modelling. For instance, we can predict a property or a structure, or we can interpret the property in terms of structure, or how a structure evolves in time, that is, the dynamics.

Historically, our research is related to the generalization of the first dimension of quantum chemistry for computations of the total energy to the second and third dimensions for computing molecular geometries and properties, respectively. This generalization is based on a mathematical machinery (called second quantization) where key formula manipulations are transferred from the wave function (or density) to the system operators, making the geometry/property calculations independent of the particular parametrization of the wave function. This led to a common formalism---and, to a large extent, also a common code--- that handles all kinds of perturbations and their associated properties.

Within the second dimension of quantum chemistry we derive structures and follow reactions, how chemical components react and pass intermediate stages to finally stabilize into products and new chemicals. Such calculations are used to understand chemical reactivity and to explore new catalysts, new reagents, biomimetic systems and how enzymes work.

Within the third dimension of quantum chemistry we consider properties that are generated by perturbing fields of any kind, from X-ray wavelengths (X-ray spectra) over UV, optical, IR, to microwave (EPR) and radio frequency (NMR) wavelengths The toolbox thus now contains up-to-date ab initio reference wave functions of variational and perturbational types with applications on molecular properties up to and including fourth order in the matter-field interaction. With the inclusion of time-dependent density functional theory into the fourth order toolbox we could enlarge the scope of applications, both with respect to the size and type of systems.

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