  We present an inexpensive first-principles approach for describing quantum
paraelectricity that combines density functional theory (DFT) treatment of the
electronic subsystem with quantum mechanical treatment of the ions through
solution of the single-particle Schr\"odinger equation with the DFT-calculated
potential. Using BaTiO$_3$, SrTiO$_3$ and KTaO$_3$ as model systems, we show
that the approach can straightforwardly distinguish between ferroelectric,
paraelectric and quantum paraelectric materials, based on simple quantities
extracted from standard density functional and density functional perturbation
theories. We calculate the influence of isotope substitution and strain on the
quantum paraelectric behavior, and find that, while complete replacement of
oxygen-16 by oxygen-18 has a surprisingly small effect, experimentally
accessible strains can induce large changes. Finally, we collect the various
choices for the phonon mass that have been introduced in the literature. We
identify those that are most physically meaningful by comparing with our
results that avoid such a choice through the use of mass-weighted coordinates.
