Improving Quantum-Chemical Methods for Molecular Properties of Complex Systems

<p><strong>Abstract</strong>:</p> <p>Electronic-structure methods have reached a state where they can be an important complement to experimental measurements. However, methods that can be used to describe the class of compounds that displays multireference character are sparse, although such compounds are often found in biological systems. Another challenge in modeling molecules from biological systems is that they are often found in a complex, heterogeneous environment, such as a protein matrix, which puts high demands to the description of the environment. Unfortunately, methods that combine a multireference method with an accurate description of a complex environment, comprising a large number of atoms, are even more rare. The developments in this thesis have made it possible to use advanced, multireference electronic structure methods for molecules in complex environments, and the methods are further extended to describe molecular excitation proles, which are important spectroscopic parameters.</p> <p>A different line of studies has focused on basis sets for molecular property calculations. These studies show for selected magnetic properties, that property specificc basis set optimization can be used to obtain results of better accuracy, than with use of the common energy-optimized basis sets. Further, a property-specificc basis set uses significantly less computational resources, to obtain a given accuracy, than an energy-optimized basis set.