ABSTRACT
As stressed throughout this book, an accurate analytical theory that allows straightforward evaluation of thermodynamic properties (such as free energy, entropy) and of microscopic observables (like radial distribution function) for many-body interacting systems (like liquids and solutions) is virtually impossible. As discussed in previous chapters, we have analytical, tractable theories only for particles interacting with simple inter-molecular model potentials. Unfortunately, for most of the liquids and solvents that we need to study (such as water), the interaction potential is far too complex for an analytical theory. Faced with such difficulties, one often attempts an alternative, direct approach to understand microscopic origin of macroscopic phenomena. Inherent structures (IS) provide such an approach. Inherent structures of a liquid are obtained by removing, in computer simulations, the vibrational degrees of freedom and by finding the nearest local potential minimum of a given microscopic state of the liquid. In principle, one can follow the trajectory of the atoms and molecules for a long time, and obtain many of these distinct inherent structures. The ensemble of these structures, along with the energy values, gives access to the non-vibrational (that is, the configurational) part of the partition function. And in the process, we get an overall view of the molecular packings in the liquid. This approach has been widely used in recent times. Among notable results, the correlation between the average energy of inherent structures and the dynamical state has been useful in many complex systems. We include this interesting topic here to make the students aware of the power of computers in pursuing non-conventional ideas.
