  We report first principles numerical discovery of hydrostatic pressure driven
tetragonal to collapsed tetragonal transition in 1111-type material ThMnAsN
accompanied by simultaneous magneto-structural, insulator to metal transition
together with complete collapse of Mn moment. We present detailed evolution of
various structural parameters, magnetism and electronic structures of ThMnAsN
with increasing hydrostatic pressure. All the structural parameters show
anomalies at a critical pressure P$_c \sim$ 9 GPa; c-lattice parameter, out of
plane As-As bond length, anion height (h$_{As}$) undergo drastic modification
compared to the in-plane parameters which is manifested in an iso-structural
phase transition from tetragonal to a collapsed tetragonal (cT) phase. These
modifications in "local structural correlations" due to pressure destroys
usually localized nature of Mn moments and gets completely quenched. Apart from
that the elastic constant, the electronic structures also bear the finger
prints of insulator-metal and magneto-structural transition at higher pressures
accompanying a total collapse of magnetic moment at the vicinity of 9 GPa. The
critical value of the pressure P$_c$ at which tetragonal to collapse tetragonal
phase transition occurs, remains robust with respect to the on-site Hubbard
correlation (U). The dynamical stability of the compound at higher pressures
are affirmed through detailed computations of phonon dispersion curves endowed
with positive phonon frequency through out the Brillouin zone. The effect of
magnetic spin structure on the electronic band structures are obtained through
band unfolding. The electronic structure of ThMnAsN at higher pressures
"orbital selectively" influences bands, band gap and closely resembles with the
electronic structure of Fe-based superconductors with the occurrences of
orbital selective Lifshitz transition.
