The Twentieth Jerusalem Symposium reflected the high standards of these distinguished scientific meetings which convene once a year at the Israel Academy of Sciences and Humanities in Jerusalem to discuss a specific topic in the broad area of quantum chemistry and biochemistry. The Twentieth Jerusalem Symposium marked an auspicious occasion. commemorating two decades of this scientific endeavour. The topic at this year's Jerusalem Symposium was Large Finite Systems which constitutes a truly interdisciplinary subject of central interest in the broad areas of chemistry. physics. astrophysics and biophysics. The main theme of the Symposium was built around bridging the gap between molecular. surface and condensed matter chemical physics. Emphasis was placed on the interrelationship between the properties of large molecules. van der Waals complexes and clusters. focusing on the structure. dynamics of nuclear motion. quantum and thermodynamics size effects. the nature of electronic states and excited-state energetics and dynamics of large finite systems. The interdisciplinary nature of these research areas was deliberated by intensive and extensive interactions between scientists from different disciplines and between theory and experiment. This volume provides a record of the invited lectures at the Symposium.