  We present an ab initio analysis for the ground-state properties of a
correlated organic compound $\kappa$-(BEDT-TTF)2Cu(NCS)2. First, we derive an
effective two-dimensional low-energy model from first principles, having
short-ranged transfers and short-ranged Coulomb and exchange interactions.
Then, we perform many-variable variational Monte Carlo calculations for this
model and draw a ground-state phase diagram as functions of scaling parameters
for the onsite and off-site interactions. The phase diagram consists of three
phases; a paramagnetic metallic phase, an antiferromagnetic (Mott) insulating
phase, and a charge-ordered insulating phase. In the phase diagram, the
parameters for the real compound are close to the first-order Mott transition,
being consistent with experiments. We show that the off-site Coulomb and
exchange interactions affect the phase boundary; (i) they appreciably stabilize
the metallic state against the Mott insulating phase and (ii) enhance charge
fluctuations in a wide parameter region in the metallic phase.
  We observe arc-like structure in Fermi surface around the region where the
charge fluctuations are enhanced. Possible relevance of the charge fluctuations
to the experimentally observed dielectric anomaly in the $\kappa$-BEDT-TTF
family compounds is also pointed out.
