 Examples of such properties are structure (i.e., the expected positions of the constituent atoms), absolute and relative (interaction) energies, electronic charge density distributions, dipoles and higher multipole moments, vibrational frequencies, reactivity, or other spectroscopic quantities, and cross sections for collision with other particles.The methods used cover both static and dynamic situations.In all cases, the computer time and other resources (such as memory and disk space) increase rapidly with the size of the system being studied.