Interatomic potential  Interatomic potentials are mathematical functions to calculate the potential energy of a system of atoms with given positions in space. Interatomic potentials are widely used as the physical basis of molecular mechanics and molecular dynamics simulations in computational chemistry, computational physics and computational materials science to explain and predict materials properties. Examples of quantitative properties and qualitative phenomena that are explored with interatomic potentials include lattice parameters, surface energies, interfacial energies, adsorption, cohesion, thermal expansion, and elastic and plastic material behavior, as well as chemical reactions. Interatomic potentials can be written as a series expansion of functional terms that depend on the position of one, two, three, etc. atoms at a time. Then the total potential of the system formula_1 can be written as   Here formula_3 is the one-body term, formula_4 the two-body term, formula_5 the three body term, formula_6 the number of atoms in the system, formula_7 the position of atom formula_8, etc. formula_8, formula_10 and formula_11 are indices that loop over atom positions. Note that in case the pair potential is given per atom pair, in the two-body term the potential should be multiplied by 1/2 as otherwise each bond is counted twice, and similarly the three-body term by 1/6. Alternatively, the summation of the pair term can be restricted to cases formula_12 and similarly for the three-body term formula_13, if the potential form is such that it is symmetric with respect to exchange of the formula_10 and formula_11 indices (this may not be the case for potentials for multielemental systems). The one-body term is only meaningful if the atoms are in an external field (e.g. an electric field). In the absence of external fields, the potential formula_16 should not depend on the absolute position of atoms, but only on the relative positions. This means that the functional form can be rewritten as a function of interatomic distances formula_17 and angles between the bonds (vectors to neighbours) formula_18. Then, in the absence of external forces, the general form becomes  In the three-body term formula_5 the interatomic distance formula_21 is not needed since the three terms formula_22 are sufficient to give the relative positions of three atoms formula_23 in three-dimensional space. Any terms of order higher than 2 are also called "many-body potentials".