Vibrational spectroscopic interests include studies of the overtone - combination spectra of crystalline hydrates at low temperatures and analyzing the data to give local mode coupling constants between OH oscillators, harmonic frequencies and anharmonicity
constants (Walker) and the application of resonance Raman spectroscopy to provide information about the vibrational coupling to electronic transitions (Lever).
We speculate that the normal Landau theory developed for 2nd order phase transition may not adequately account for the physics of the phase transitions and associated phenomena, for example, magneto-volume effect due to lattice anharmonicity
in Fe pnictide superconductors.
These properties are: time-independent behavior, reversible dynamics, thermal statistical equilibrium and irreversible evolution toward equilibrium, together with anharmonicity
and anharmonic couplings.
The higher order terms allow to describe these effects in a more general way, so their sign and values must agree with the idea that the global consequence of anharmonicity
is to lower the potential energy of oscillation; indeed the potential energy reads [a'.