  We report on the microscopic magnetic modeling of the spin-1/2 copper mineral
malachite at ambient and elevated pressures. Despite the layered crystal
structure of this mineral, the ambient-pressure susceptibility and
magnetization data can be well described by an unfrustrated
quasi-one-dimensional magnetic model. Weakly interacting antiferromagnetic
alternating spin chains are responsible for a large spin gap of 120K. Although
the intradimer Cu-O-Cu bridging angles are considerably smaller than the
interdimer angles, density functional theory (DFT) calculations revealed that
the largest exchange coupling of 190K operates within the structural dimers.
The lack of the inversion symmetry in the exchange pathways gives rise to
sizable Dzyaloshinskii-Moriya interactions which were estimated by
full-relativistic DFT+U calculations. Based on available high-pressure crystal
structures, we investigate the exchange couplings under pressure and make
predictions for the evolution of the spin gap. The calculations evidence that
intradimer couplings are strongly pressure-dependent and their evolution
underlies the decrease of the spin gap under pressure. Finally, we assess the
accuracy of hydrogen positions determined by structural relaxation within DFT
and put forward this computational method as a viable alternative to elaborate
experiments.

Strongly Correlated Electrons