  We investigate the effect of epitaxial strain on the Mott metal-insulator
transition (MIT) in perovskite systems with d^1 and d^2 electron configuration
of the transition metal (TM) cation. We first discuss the general trends
expected from the changes in the crystal-field splitting and in the hopping
parameters that are induced by epitaxial strain. We argue that the
strain-induced crystal-field splitting generally favors the Mott-insulating
state, whereas the strain-induced changes in the hopping parameters favor the
metallic state under compressive strain and the insulating state under tensile
strain. Thus, the two effects can effectively cancel each other under
compressive strain, while they usually cooperate under tensile strain, in this
case favoring the insulating state. We then validate these general
considerations by performing electronic structure calculations for several d^1
and d^2 perovskites, using a combination of density functional theory (DFT) and
dynamical mean-field theory (DMFT). We isolate the individual effects of
strain-induced changes in either hopping or crystal-field by performing DMFT
calculations where we fix one type of parameter to the corresponding unstrained
DFT values. These calculations confirm our general considerations for SrVO_3
(d^1) and LaVO_3 (d^2), whereas the case of LaTiO_3 (d^1) is distinctly
different, due to the strong effect of the octahedral tilt distortion in the
underlying perovskite crystal structure. Our results demonstrate the
possibility to tune the electronic properties of correlated TM oxides by using
epitaxial strain, which allows to control the strength of electronic
correlations and the vicinity to the Mott MIT.
