Is heating from the ground state of motion the same thing as decoherence?
Yes. When starting from the ground state of motion, heating to excited states involves a evolution in the vibrational state population. However, because the initial pure quantum state evolves into a mixed state, where it is not possible to speak of a wavefunction anymore, this is indeed decoherence. While in the basis of vibrational states there are never any coherences (eg, off-diagonal matrix elements of the density matrix), the fact remains that this is a question of density matrix basis. In other bases, the initial ground state can be expressed as superpositions of eigenstates — such as the Gaussian wavepacket in position space, and the same process of incoherent heating will certainly destroy any coherences in this representation. Decoherence does not depend upon the particular basis chosen to represent the quantum state!
