  Charged defects in 2D materials have emerging applications in quantum
technologies such as quantum emitters and quantum computation. Advancement of
these technologies requires rational design of ideal defect centers, demanding
reliable computation methods for quantitatively accurate prediction of defect
properties. We present an accurate, parameter-free and efficient procedure to
evaluate quasiparticle defect states and thermodynamic charge transition levels
of defects in 2D materials. Importantly, we solve critical issues that stem
from the strongly anisotropic screening in 2D materials, that have so far
precluded accurate prediction of charge transition levels in these materials.
Using this procedure, we investigate various defects in monolayer hexagonal
boron nitride (h-BN) for their charge transition levels, stable spin states and
optical excitations. We identify $C_BN_V$ (nitrogen vacancy adjacent to carbon
substitution of boron) to be the most promising defect candidate for scalable
quantum bit and emitter applications.
