  Rare-earth monopnictides have attracted much attention due to their unusual
electronic and topological properties for potential device applications. Here,
we study rock-salt structured lanthanum monopnictides LaX (X = P, As) by
density functional theory (DFT) simulations. We show systematically that a
meta-GGA functional combined with scissor correction can efficiently and
accurately compute electronic structures on a fine DFT $k$-grid, which is
necessary for converging thermoelectric calculations. We also show that strain
engineering can effectively improve thermoelectric performance. Under the
optimal condition of 2% tensile strain and carrier concentration
$n=3\times10^{20}~\textrm{cm}^{-3}$, LaP at temperature 1200 K can achieve a
figure of merit $ZT$ value $>2$, which is enhanced by 90% compared to the
unstrained value. With carrier doping and strain engineering, lanthanum
monopnictides thereby could be promising high-temperature thermoelectric
materials.
