A half-filled-band Hubbard model on an anisotropic triangular lattice (t in
two bond directions and t' in the other) is studied using an optimization
variational Monte Carlo method, to consider the Mott transition and
superconductivity arising in \kappa-BEDT-TTF_2X. Adopting wave functions with
doublon-holon binding factors, we reveal that a first-order Mott
(conductor-to-nonmagnetic insulator) transition takes place at U=U_c
approximately of the band width, for a wide range of t'/t. This transition is
not directly connected to magnetism. Robust d-wave superconductivity appears in
a restricted parameter range: immediately below U_c and moderate strength of
frustration (0.4\lsim t'/t\lsim 0.7), where short-range antiferromagnetic
correlation sufficiently develops but does not come to a long-range order. The
relevance to experiments is also discussed.Comment: 15 pages, 17 figures, submitted to J. Phys. Soc. Jp