  Precise fundamental atmospheric stellar parameters and abundance
determination of individual elements in stars are important for all stellar
population studies. Non-Local Thermodynamic Equilibrium (Non-LTE; hereafter
NLTE) models are often important for such high precision, however, can be
computationally complex and expensive, which renders the models less utilized
in spectroscopic analyses. To alleviate the computational burden of such
models, we developed a robust 1D, LTE and NLTE fundamental atmospheric stellar
parameter derivation tool, $\texttt{LOTUS}$, to determine the effective
temperature $T_{\mathrm{eff}}$, surface gravity $\log g$, metallicity
$\mbox{[Fe/H]}$ and microturbulent velocity $v_{\mathrm{mic}}$ for FGK type
stars, from equivalent width (EW) measurements of Fe I and Fe II lines. We
utilize a generalized curve of growth method to take into account the EW
dependencies of each Fe I and Fe II line on the corresponding atmospheric
stellar parameters. A global differential evolution optimization algorithm is
then used to derive the optimized fundamental parameters. Additionally,
$\texttt{LOTUS}$ can determine precise uncertainties for each stellar parameter
using a Markov Chain Monte Carlo (MCMC) algorithm. We test and apply
$\texttt{LOTUS}$ on a sample of benchmark stars, as well as stars with
available asteroseismic surface gravities from the K2 survey, and metal-poor
stars from $R$-process Alliance (RPA) survey. We find very good agreement
between our NLTE-derived parameters in $\texttt{LOTUS}$ to non-spectroscopic
values within $T_{\mathrm{eff}}=\pm 30$ K and $\log g=\pm 0.10$ dex for
benchmark stars. We provide open access of our code, as well as of the
interpolated pre-computed NLTE EW grids available on Github, and documentation
with working examples on Readthedocs.
