PROPERTIES OF ATOMS IN MOLECULES - ATOMS UNDER PRESSURE

The in situ pressure acting on the surface of an open system at the at omic level is defined and determined by the virial theorem for a prope r open system, one whose spatial boundary and equations of motion are determined by the principle of stationary action. The quantum pressure is determined by the virial of the force resulting from the electroni c momentum flux through the surface of the open system. A scaling proc edure is used to demonstrate that the expectation value of the pressur e-volume product of a proper open system is proportional to its surfac e virial. Previous work, in analogy with the classical virial theorem for a contained system, incorrectly relates the pressure to the extern al forces of constraint acting on a closed system. A neon vise consist ing of a chain of three, four or five hydrogen molecules compressed be tween two neon atoms is used to introduce the quantum definition of pr essure and study its effect on the mechanical properties of an atom an d on the topology of the electron density. Pressures approaching 160 G Pa have been calculated for the vise. The topology of the electron den sity and the homeomorphism it exhibits with the virial field are found to be invariant to an increase in pressure, the electron density accu mulating to an ever increasing extent between all pairs of nuclei whic h serve as the sole attractors. The virial of the Ehrenfest force acti ng on the surface of a compressed molecule provides a measure of the i ncrease in the electronic kinetic energy resulting from the applied pr essure. The effects of pressure on the intra- and intermolecular bondi ng are discussed in terms of pressure-induced changes in the electron density and in the mechanical properties of the atoms. (C) 1997 Americ an Institute of Physics.