THEORY OF RESPONSE PROPERTIES OF ATOMS IN MOLECULES

A general theory of response properties of atoms in molecules is prese nted. New concepts of atomic projection and atomic perturbation operat ors lead to a formalism for the calculation of response properties of arbitrary order. The atomic Hellmann-Feynman theorem is formulated for the first-order properties. The second-order properties of atoms in m olecules are given by symmetrized sums of contributions due to atomic pairs, which can be computed with ordinary second-order perturbation t heory. The expressions incorporate the effects of perturbations on the atomic zero-flux surfaces through the use of relaxed atomic perturbat ion operators. For this reason, unlike those arising from previously p roposed definitions, the resulting higher-order response properties ar e invariant to interchanges between the perturbation operators, and do not involve separate basin and surface terms.