  Electronic, structural, vibrational and elastic properties of PaN have been
studied both at ambient and high pressures, using first principles methods with
several commonly used parameterizations of the exchange-correlation energy. The
generalized gradient approximation (GGA) reproduces the ground state properties
satisfactorily. Under pressure PaN is found to undergo a structural transition
from NaCl to the R-3m structure near 58 GPa. The high pressure behavior of the
acoustic phonon branch along the (1,0,0) and (1,1,0) directions, and the C44
elastic constant are anomalous, which signals the structural transition. With
GGA exchange-correlation, a topological transition in the charge density occurs
near the structural transition which may be regarded as a quantum phase
transition, where the order parameter obeys a mean field scaling law. However,
the topological transition is absent when other exchange-correlation
functionals are invoked (local density approximation (LDA) and hybrid
functional). Therefore, this constitutes an example of GGA and LDA leading to
qualitatively different predictions, and it is of great interest to examine
experimentally whether this topological transition occurs.
