Van der Waals (vdW) interactions are ubiquitous in nature, playing a major role in defining the structure, stability, and function for a wide variety of molecules and materials. VdW interactions between atoms are weak (typically 0.01-0.1 eV), however the vdW energy scales linearly with system size and renders a large contribution to intermolecular and conformational energies. I will review the state-of-the-art of theoretical methods for the description of vdW interactions, ranging from empirical treatment in force fields to accurate ab initio calculations. Our recent developments for including the full long-range treatment of vdW energy accurately and efficiently will be presented [1,2,3,4]. Finally, I will show that vdW interactions are crucial for shaping the secondary structure of peptides and proteins, explaining the extreme thermal stability of polyalanine helices in vacuo .
 A. Tkatchenko and M. Scheffler, Phys. Rev. Lett. 102, 073005 (2009).
 A. Tkatchenko, R. A. DiStasio, Jr., M. Head-Gordon, M. Scheffler, J.
Chem. Phys. 131, 094106 (2009).
 X. Ren, A. Tkatchenko, P. Rinke, M. Scheffler, Phys. Rev. Lett. 106, in print (2011).
 A. Tkatchenko, R. A. DiStasio, Jr., R. Car, M. Scheffler, in preparation.
 A. Tkatchenko, M. Rossi, V. Blum, J. Ireta, M. Scheffler, Phys. Rev.
Lett. 106, 118102 (2011).
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