Adjective
Not harmonic.
Source: Webster's dictionaryHe then solved the same problem by treating the anharmonic potential term as a perturbation to the harmonic oscillator and using the perturbation methods that he and Born had developed. Source: Internet
Describing of anharmonic effects is complicated because exact treatment as in the harmonic case is not possible and phonons are no longer exact eigensolutions to the equations of motion. Source: Internet
The two most important anharmonic effects are the thermal expansion and the phonon thermal conductivity. Source: Internet